Comment 13 for bug 312319

Found this:

http://www.rikertothebridge.com/2009/04/xorg-server-fails-to-start-on-ubuntu-with-more-than-one-nvidia-gfx-card-in-machine/

Adding a "Busid" line to the Device section of xorg.conf lets me start the X server

lspci lists things like this:

"lspci" lists the graphics cards:
01:00.0 VGA compatible controller: nVidia Corporation Quadro FX 570
02:00.0 VGA compatible controller: nVidia Corporation Quadro FX 570

The corresponding Busid lines would be:

Busid "PCI:1:0:0"

or

Busid "PCI:2:0:0"

Changing xorg.conf to add the Busid gave me a mirrored desktop on two monitors.

After this I tried to run nvidia-xconfig, but this resulted in a blank screen on attempting to restart the X server.

nvidia-xconfig tries to load the "nvidia" driver, but this fails. I had to revert to the "nv" driver.