Alternately, one could go with just one very high gain screen if you want larger 3D images, or larger 2D for that matter. The usual tradeoff with the really high gain screens, however, is that they offer a limited viewing angle, and you are far more likely to notice differences in brightness as the eye moves from the center (or hot spot) of the screen out to the edges. With standard gain screens from about .8 to 1.5, these types of things are pretty minimal but not so when you go higher. I like the Mitsubishi HC7800 for 3D, quite a bit, as long as one keeps the screen size and the room setup such that you have adequate brightness to really enjoy it.
Perhaps the best one screen solution for the HC7800D, would be to just pick out the best screen for your room and 2D viewing. Have your projector mounted so you can zoom the lens smaller (really tricky if you have a high ceiling, of course). Then, when you want to watch 3D, zoom out and reduce the image size to, say 80" diagonal hand have a reasonably bright image.
My only problem with that, is, nothing cries out for a bigger image - not a smaller one - than 3D. 42" LCDTVs are so small, they almost make a mockery of 3D, compared to 100" diagonal!