While two projectors would be cool, it would be a pain to keep them both calibrated. For "pure 3D", they both need to be setup the exactly the same (white point, brightness, etc.).
The lamp will dim at different rates between the two. The difference in time of the lamp dimming will also cause an uneven white point change (unless you keep up on making sure they are both 6500k). The iris's may behave slightly differenty, even in the same shots. In the LCDs, one of them might develop a color uniformity issue (very likely if epsons are used...I've had 5 of them). If there is no manual iris, you'll have a tough time getting the brightnesses matching. The key is to keep both EXACTLY the same - not an easy task.
Although, if you had 2 LED projectors, or two laser projectors, consistency should theoretically not be an issue, especially if using a manual iris to match the brightness.
I'm not trying to discourage - I've even thought about it myself. BUT, I then started to think about all the stuff I just mentioned, so I never personally did it.