2 720P projectors doesn't even give you a decent resolution however.
If you are going for multiple projectors to improve brightness etc, surely higher resolution would also be a useful point.

Just an idea as well, but if you have 2 DLP 3D projectors, can you double them up on the imagery and gain 2x brightness that way? If you had the correct HDMI splitters etc would they switch in phase with each other? if so, that would probably be the best solution for both brightness AND useability, since you could split them into a 2 projector 3D setup, or align them for a high brightness 720P 3D setup.

The problem is that consumer cards don't offer the frame-lock feature that professional cards have. Because of this lack of frame-lock, multiple outputs don't refresh at the same time which leads to the 1-frame-lag issue that affects all multi-output solutions on consumer cards.
When both eyes aren't refreshed at the same time, it creates eye strain as well as a motion distortion artefact which modifies the horizontal parallax when there is horizontal motion (changes depth, can lead to depth violation) and creates vertical parallax when there is vertical motion (eyestrain).

This also happens with frame sequential displays, but the amount of the effect is not the same.
Frame sequential displays typically work at 120Hz, so the lag between the eyes is 1/120 of a second, all the time.

With non-synchronized dual-outputs displays, the lag is variable, let's assume we're using 60Hz displays (iZ3D monitor or projectors)
First there is the base lag due to the lack of display sync : it's a random amount of lag which can vary between 0 second and 1 refresh period = 1/60 of a second

Then there is the frame rate issue, in games the 1-frame-lag kicks in, the primary display get's it's picture as soon as possible or at the first refresh available if v-sync is enabled, but the secondary display only gets it's picture on the next frame. I don't know why (probably something to do with the way Windows and DirectX work), but it's just happens like that on every graphics card I've tested. (Geforce 8800GTX, Radeon HD5870, HD6850, HD7970)
The lag created varies according to the frame rate.
-if the game has a significantly higher frame rate than the refresh rate (more than 60fps), there is a chance that the next frame will be completed before the refresh sync and both pictures will be displayed at the same time (in addition to the unsync base lag)
-if the game runs at 60fps, the 1 frame lag will make the picture for the secondary display too late to catch the refresh sync and the picture will be displayed with 1/60 of a second lag (in addition to the unsync base lag)
-if the game runs at 30fps, the 1 frame lag will make the picture for the secondary display too late by twice the refresh sync and the picture will be displayed with 1/30 of a second lag (again in addition to the unsync base lag)
-etc... 20fps -> 3 times the sync -> 1/20 s lag, etc...

For static scenes there's no problem, but when things are in motion, these lags become significant problems.
The iZ3D monitor has an advantage, because of the special colour + polarisation pixel format, the 1-frame-lag would not cause eye disparity, but dual projectors get hit head on with it one eye sees a completely different picture than the other eye : when I get that much lag, I just can't watch the picture : it hurts my eyes too much.

The only way to counter this is to make the graphics card manage both outputs as one single display at a low level (not just emulating an extended/spanned desktop)
AMD Eyefinity allows this under very tight restrictions (exact identical outputs : resolution, framerate and cable/adapters) and that's the way I do dual-projectors
Nvidia Surround does NOT allow it at all (Nvidia surround (2D or 3D) requires 3 displays)
An other way to do it would be to use a Matrox Dualhead2go, but it's only theory, I haven't read any reports of people testing this solution. I do not know exactly how the Matrox drivers interact with Windows or games, so I do not know if this solution works until someone can test it.

I think salvation for dual-projectors would have to come in the form of an active converter capable of transforming HD3D or 3DTV Play inputs (Hdmi 1.4 3GHz or DisplayPort 1.2) and convert them to Left/Right dual head outputs (dual DVI or dual hdmi).
Such conversion is currently possible for hdmi1.4a (minimum spec) but is expensive : it requires 3 devices (1 hdmi1.4a splitter and 2 Optoma 3DXL).
There is currently no such solution supporting stereo 1080p60.

My bad. i was reading about the new LG 80" class 3D TVs and thought they were out already. the 65lm6200 is a 65" that can be had for about $2000.

Have you checked the forum over at stereo3d.com some of those guys have some serious projection setups.
John

I have been playing UT3 , Portal 2 and CoD on my demo rig. i know its PC based and we were first aiming at movies in passive 3D with the system but im a long time gamer and this thing rocks on UT3.
the quadro is not my first choice in cards to make a dual projector passive 3d rig but its what i had on hand. my home gaming system is way better and im planning on building a new PC and setup some spare DLP rigs in my home office with a curved 96" screen. i finnaly found some decent screen warping software to work with windows. ive got 2 1080P DLP projectors and i will have to see how smooth it is.