The purpose for image stacking is to overlap two projector images together to double the brightness. The cost for two 5000 Lum projector stacking to get 10,000 Lum will be much cheaper than one 10,000 Lum projector and the result is similar.
Another typical application is passive 3D application. Passive 3D requires two projectors and these two projectors needs to be stacked together. One projector displays RH eye image and another one displays LH eye image. After the light paths go through polarized filter and wear polarized 3D glasses, viewers will see comfortable 3D image.