I currently run a quad monitor array two monitors on each GPU(2x Nvidia 8800 Ultras). I run my setup using Xinerama with independent x screens. I have recently gotten a bug in my ass to get compiz working. That obviously requires compositing to be enabled which Xinerama disables. There use to be a way a while back to use XGL to get this to work, however, development was stopped on that a while back. There is now AIGLX which, according to reading, NVIDIA's proprietary drivers have their own implementation of it. I read someone a few months ago there is a way to tie together two GPUs running TwinView using randr. It would work as I could use Fake Xinerama but, I've also read that there isn't even multi-GPU support in randr yet as of 1.3. If someone could confirm this method that would be great.
If anyone has ideas, infomation, and/or comments please post.