So I’ve checked around and I’ve gotten mixed messages on this issue. I currently have a 5 monitor setup across 3 GPU cards, each of them being dual head cards. I can get it to work with Xinerama but the performance is terrible. I have used xrandr in single GPU configurations before with great success, but I’m not sure if I can do this with multiple GPUs. Just checking to see if I’m missing something here. Thanks.
I can’t answer your question as well as I’d like to, since I’ve never owned a multiple GPU card. I’m assuming you’re using NVIDIA cards, correct? (You should really post these details and exact setup as it may assist others who could possibly help here). AFAIK, the proprietary nvidia driver doesn’t fully support the xrandr extensions yet, even though RandR 1.3 might provide some support for multiple screens/cards.
An interesting Ubuntu thread discussing this subject.