Re: problems with compiz and multiple cards
Even if it were somehow possible (perhaps using XGL but I have my doubts about it) to use a 'xinerama'-like setup using two cards the performance would suck especially in cases where you have to move windows between the two displays. The main performance issues of compiz/kwin are related to copying pixmaps (windows, bitmaps) from system memory to video memory 'when needed'. This is quite complicated as you don't want to move pixmaps around all the time as it is slow, when it is in video memory you want to keep it there. When you move windows or so between two displays in a setup with two cards you have to a lot of moving from card 1 to card 2 which is very inefficient and this kills performance.
I believe on Windows Nvidia offers these days twinview in sli mode. If that's correct they likely didn't take the time to port it to Linux yet (read no big customers asked for it).