I recently ran across a problem on my workstation where i've tried get two Nvidia 9500GT cards to run together so i could attach 3 screens (two on the primary card and one on the third card). Sometimes the third screen worked and sometimes it didn't. The reason for this was, that as it seemed the nvidia driver for the first screen took away all available virtual memory so that the second driver did not have enough to start. this produced the following errors in /var/log/Xorg.0.log:
Failed to initialize the NVIDIA graphics device PCI:5:0:0. Please see the COMMON PROBLEMS section in the README for additional information. Failed to initialize the NVIDIA graphics device!
the solution to this problem is, to reserve some virtual memory when booting the os so there will enough be available later on (don't ask me how it acutally works, i just found this out by reading some suggestions in forum posts on the web) .. to do so add this to the end of your kernel options line in your boot manager (usually after “quiet splash” or something like that)
this did the trick for me :) maybe you need to play around with the size of the reserved memory..