Originally Posted by Thunderbird
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).
Just for the record I do have the two cards and monitors setup in xinerama. As you said dragging between screens does cause smearing and is probably not the greatest. What i am mainly worried about though is performance once the application is on one screen or another. I'm planning to overclock the pcix to maybe get a little bit better performance between monitors. I'm hoping to see twinview in SLI soon as this is what i wanted to originally do. Eventially though i want to get a quadro because it has better linux support from what I've seen and will give me better performance for my work. Also does anyone know if twinview will be in Linux on SLI ever?