DUAL PROJECTION DEVICE AND METHOD Technical Field [1] The present invention relates to a dual projection device and more particularly to a dual projection device comprising first and second projectors that switches on and off by turns to project images onto a screen and a method for performing dual projection by switching on and off the first and second projectors by turns. [2] Projection device (or projector) is connected to a image signal providing apparatus such as VCR, DVD player, laptop computer, PC to transform the image signals from the image signal providing apparatus into visible light that is to be projected onto a image screen thereby enabling users to recognize the images. Background Art [3] Conventional projector system comprises a single projector 111 for projecting the image to a screen 110 in a predetermined distance. The projector 111 is connected to an image providing apparatus (not shown) such as VCR, DVD player, laptop computer, PC, etc. [4] The projector 111 generates a lot of heat during it is turned on and projects image onto the screen. In order to solve the heat problem the conventional projection system would contain a cooling fan inside the projector 111 or an external air conditioner operated to cool down the projector 111. [5] The projection system usually operates for 1 to two or 3 to 4 hours continuously for a conference, seminar, presentation, and visual education purposes in a conference room of a corporation or a lecture room in an educational facility. [6] That is, it is not usual to continuously use the projector 111 for very long time and even when the projector 111 is extraordinarily used for a long time it is necessarily cooled down by temporarily switching off the power of the projector for a pre¬ determined time (i.e. during intermission). Disclosure of Invention Technical Problem [7] The present invention addresses the problem of heat generation when the projection system is continuously used for an extraordinarily long time. [8] For example, in an important meeting of a long time or in a game room where the projector needs to be continuously operated very long time, the turn-off of the projection system may cause problems of unexpected interruption during a meeting and discontinuity of a gaming process. [9] The purpose of the present invention is to resolve the above problem and to provide a novel projection system that could be used for a very long time without interruption during the operation. Technical Solution [10] The dual projection device of the present invention comprises a first projector for projecting images onto a screen; a second projector for projecting images onto the screen; and a control means for enabling the first and second projectors to project images onto the screen by turns by switching on and off the first and second projectors by turns. [11] The method of dual projection of the present invention comprises steps of enabling a first projector to project images onto a screen while a second projector is switched off for a first time period; enabling the second projector to project images onto the screen while the first projector is switched off for a second time period; and pe¬ riodically repeating the above two steps by turns. [12] Preferably, both the first and the second projectors are switched on for a pre¬ determined time period at the end of the first time period and the beginning of the second time period and, during the predetermined time period, the brightness of the image projected by one projector increases while the brightness of the image projected by the other projector decreases. Advantageous Effects [13] According to an aspect of the present invention, since each of the two projectors projects images onto the same screen for a predetermined time one projector could be switched off and cooled down while the other one operates to continuously provide visual images for a long time without interruption during the use of the projection system. [14] According to another aspect of the present invention, since both the first and the second projectors are switched on for a predetermined time period at the end of the first time period and the beginning of the second time period and the brightness of the image projected by one projector increases while the brightness of the image projected by the other projector decreases during the predetermined time period, the projection system could maintain the brightness of the image of a normal operation even when the first and second projectors are switched on and off, and thereby enables the viewers to see the image without recognizing the switching on and off of the projectors. Brief Description of the Drawings [15] Fig. 1 is an outline view of a conventional projection system. [16] Fig. 2 is an outline view of a dual projection system of the present invention. [17] Fig. 3 is a perspective view of a dual projection system of an embodiment of the present invention. [18] Fig. 4 is cross-sectional view of a dual projection system of an emboliement of the present invention. [19] Fig. 5 is an outline view of a dual projection system of another embodiment of the present invention. [20] * Reference numbers in the drawings * [21] 110, 210: screen 111, 211, 311: first projector [22] 212, 312: second projector 320: retaining housing [23] 330: supporting member 400: cooler [24] 410: air inlet 420: air cleaning room [25] 430: air cleaner 440: airway [26] 450: cooling fan 460: air outlet Best Mode for Carrying Out the Invention [27] The preferred embodiment of the present invention will be described in detail referring to the accompanied drawings. [28] Fig. 2 is an outline view of a dual projection device of the present invention. The dual projection device of the present invention comprises a first projector 211 for projecting images onto a screen 210; a second projector 212 for projecting images onto the screen 210; and a control means (not shown) for switching on and off the first and second projectors 211, 212 by turns. [29] The control means can be an embedded type that can be installed inside the first and second projectors 211, 212. It can be an external type that can be connected between the first or second projector 211, 212 and the image providing apparatus such as laptop computer and personal PC. The control means can be a hardware embedded in a storage device of a laptop computer or personal PC. When the control function is provided by software the laptop computer or personal PC itself can be the control means. [30] In an exemplary embodiment of the method of dual projection of the present invention, the first projector 211 projects images onto a screen 210 for a first pre¬ determined time and then switches off and the second projector 212 switches on when or before the first projector 211 switches off. Then the second projector 212 projects images onto the same screen 210 for a second predetermined time and switches off. This procedure repeats itself during the operation of the dual projection device. When the dual projection device allows the selection of dual mode operation and single mode operation the above procedure repeats when the dual projection mode is selected. [31] Preferably, both the first and the second projectors 211, 212 could be switched on for a predetermined time period when the first and second projectors are switched on and off by turns. More specifically, both the first and the second projectors 211, 212 could be switched on for a transition period between the end of the working time of the first projector 211 and the beginning of the working time of the second projector 212. The projection device could be controlled by the control means to increase the brightness of the image projected by one projector while decreasing the brightness of the image projected by the other projector, thereby maintaining the brightness of the overall image from the projection device regardless of the existence of the transition period. [32] The time period of switching on the projectors 211, 212 could be controlled based on the amount of heat generated by the projectors 211, 212. For example, the control means (not shown) can control the first and second projectors 211, 212 to be switched on and off by turns once in an hour. [33] Fig. 3 is a perspective view of the dual projection device according to an embodiment of the present invention. The first and second projectors 311, 312 are mounted within the retaining housing 320 and the second projector 312 is located under the first projector 311. The retaining housing 320 holds the first and second projectors 311, 312 so that images from the first and second projectors 311, 312 can be projected onto the same position on the same screen 210. While the retaining housing 320 for holding the first and second projectors 311, 312 can be mounted on a table for example, it 320 can also be fixedly installed on the floor, ceiling or wall with the help of supporting member 330. [34] Meanwhile, in addition to the fastening function given by the retaining housing 320 for projecting images to the same position, the first and second projectors 311, 312 can be manipulated to project images to the exactly same position by an image-control means such as pivot axis. The projection point can also be finely controlled by software tools during the use or installation of the dual projection device. [35] Further, the retaining housing 320 is preferably equipped with a cooler 400 to which the cooling parts (i.e. a cooling part including cooling fans of the projector) of the first and second projectors 311, 312 can be inserted. [36] The cooler 400 will be described in more detail with reference to Fig. 4. [37] The cooler 400 has air inlet 410 for intaking outside air so that the outside air flows into the air cleaning room 420 through the air inlet 410. An air cleaner 431 is provided within the air cleaning room 420. Although not illustrated for simplicity, the cooling parts of the first and second projectors 311, 312 are inserted into the air cleaning room 420. The air cleaner 431 according to a preferred embodiment of the present invention has for filters: a pre-filter 431 (i.e. first filter) for filtering out large scale dusts, a medium-filter 432 (i.e. second filter) for filtering out normal scale dusts, an activated carbon filter 433 (i.e. third filter) for absorbing noxious gases and bad smells, and a HEPA (High Efficiency Particulate Air) filter 434 (i.e. fourth filter) for removing fungus, bacteria and nicotine. [38] The four filters purify the outide air that entered into the air cleaning room 430. Then the air passes along the airway 440 to be exhausted at the air outlet 460. Cooling fan 450 connected to a motor 451 is provided to generate airflows. According to the above-mentioned structure, the cooler 400 performs air-cooling operating by exhausting the air heated by heats from the projectors 311, 312. The cooler 400 also performs air-cleaning operation by the air cleaner 431. Mode for the Invention [39] Fig. 5 is a dual projection device according to another embodiment of the present invention. [40] In this embodiment, one large screen is virtually divided by N (N is natural number equal to or lager than 2) small screens. A first projector group consisting of N projectors and a second projector group consisting of different N projectors switch on and off by turns to project images to corresponding screens to form a larger single image. [41] Fig. 5 illustrates a projection system wherein N=3 and will be described in more detail. [42] The proj ector ( 1-1) 511 and the proj ector(2- 1) 512 proj ect image onto the first screen 510, the proj ector( 1-2) 521 and the projector(2-2) 522 project image onto the second screen 520, and the projector(l-3) 531 and the projector(2-3) 532 project image onto the third screen 530. [43] One single total image is divided into three sub-images and provided to the cor¬ responding projectors so that the respective images projected onto the first, the second and the third screens 510, 520, 530 are combined to form a single originally given image. [44] The projector(l-l) 511, the proj ector( 1-2) 521 and the projector(l-3) 531 form one projector group (i.e. the first projector group), and the proj ector (2-1) 512, the projector(2-2) 522 and the projector(2-3) form another projector group (i.e. the second projector group). [45] The first and the second projector groups are controlled by a control means to be switched on and off to project images onto the screen by turns. That is, the first projector group consisting of the projector(l-l) 511, the proj ector( 1-2) 521 and the projector(l-3) 531 projects images onto the respective screens 510, 520, 530 for a first predetermined time and then switches off. The second projector group consisting of the proj ector (2-1) 512, the projector(2-2) 522 and the projector(2-3) 532 switches on when or before the first projector group switches off. The proj ector (2-1) 512, the projector (2-2) 522 and the projector(2-3) 532 project images onto the screens 510, 520, 530 re- spectively for a second predetermined time and then switches off. Then the first projector groups turns on again. This procedure repeats itself during the operation of the dual projection system. When the dual projection system allows the selection of dual mode operation and single mode operation the above procedure repeats when the dual projection mode is selected. [46] Preferably, both the first and the second projector groups could be switched on for a predetermined time period when the first and second projector groups are switched on and off by turns. More specifically, both the first and the second projector groups could be switched on for a transition period between the end of the working time of the first projector group and the beginning of the working time of the second projector group. [47] Although the present invention has been explained with reference to the drawings, the present invention shall not be limited by the above-explained embodiments. Various modifications can be accomplished by those skilled in the art without departing from the scope of the present invention defined by the appended claims. Industrial Applicability [48] The dual projection device of the present invention can be used in an important meeting of a long time or in a game room where the projector needs to be continuously operated very long time. Since multiple projectors of the dual projection device project images onto the same screen by turns, one projector may be switched off and cooled down while the other projector is in operation and thus the dual projection device of the present invention can provide images continuously for a long time. [49] Especially the projection scheme of dividing the total image into N sub-images and then projecting onto respective portion of a large screen can be usefully employed in a game room for horse racing game and bicycle racing game, etc.