after searching I have not found a solution. I have 2 nvidia quadro 295 cards with 3 monitors but I can not extend twinview across multiple GPUs. I have the most recent nvidia proprietary drivers installed, and they work fine for the single GPU / dual monitors. I tried using xinerama, but without success. I don't mind switching to the nouveau drivers if that would handle my multiple monitors, although I would like to continue to use unity and compiz if i can).