Originally Posted by MTyson
Actually it's the worst way. You're eyes will adjust to the brightest images whites. Not only that, but if one has a significant black level, contrast or shadow detail advantage those advantage will diminish due to the light washout caused by the other projector. The test would be fine with them in the same room and on at the same time as long as you put the lens cap on one of the units while you test the other one.
The same goes for screen comparisons. It took me awhile, but I learned it doesn't work right to test them at the same time. If you test on both a white and light grey screen the white screen's whites will make the grey screen's whites look poor, yet if you viewed the light grey screen by itself the whites will most likely look great This because your eyes are adjusted only for its whites, therefor your perception of everything is not changed with the introduction of a brighter white.
This can be proven even futher by stacking my silver torus against a 1.3 gain white screen. What happens when viewed at the same time? Well, the white piece looks about 10x darker and the whites look dark grey in comparison. By itself though the 1.3 gain white doesn't look like that though.
If you just want to review projectors, I think your theory makes sense. But for determining which projector you want to buy, I'd rather have them side by side.