In Fedora 11 (64bit) I was using the nvidia binary drivers (akmod-nvidia) to drive both by QUadro FX 1400, with 2 monitors on it, and my GeForce 8400 GX, with 1 monitor on it.
After upgrading to Fedora 12 I found that that setup would make the whole machine crash, it just hung, after about 5mins of use. The only way to fix this was to go to nvidia-settings and disable the FX 1400, that is, disable the third monitor.
I have not been able to find any reports of others with similar experience, but then again, it seems that no one is willing to put 2 video cards on a Linux machine.
As there any workarounds?