Sounds like you're using the proprietary Nvidia driver?
Set all your screens to 'separate X screen' and then check the 'Use Xinerama' box. You may have to set the correct screen offsets in the Nvidia control panel (or xorg.conf) after restarting X.
I have this setup at work, and it works very nicely apart from the fact that only one of the two GPUs seems to be capable of displaying OpenGL content when connected in this way.
Yes, I'm using the proprietary drivers (for a variety of reasons, but CUDA support chief among them) and I had that exact setup for about five minutes.
Using Xinerama forces compositing off, which makes for a less than snappy experience. I have three Dell 3000wfp's and without compositing turned on, the tearing is more than noticeable. I might be able to tolerate it, but I know that it is capable of working nicely (a la TwinView).
Doesn't exist (yet). The intended replacement is multi-gpu support in xRandR but that feature has been pushed back release after release of xRandR for literally years now.
Set all your screens to 'separate X screen' and then check the 'Use Xinerama' box. You may have to set the correct screen offsets in the Nvidia control panel (or xorg.conf) after restarting X.
I have this setup at work, and it works very nicely apart from the fact that only one of the two GPUs seems to be capable of displaying OpenGL content when connected in this way.