I just was thinking, that you should make sure that your two cards are not setup for SLI. SLI will supposedly not work for what you are trying to accomplish.
Also, once you have made sure that nvidia and not nouveau is the only video driver:
in /etc/modprobe.d/fbdev-blacklist, add:
~# sudo update-initramfs
~# sudo update-grub #or what update what bootmanager you use
to make sure modules aren't loaded from the initramfs.
Then you can reboot into single user mode to have a fresh environment (this will be as root); run:
~# Xorg -configure
~# nvidia-xconfig -f ./xorg.conf.new -o ./xorg.conf
Review the resulting xorg.conf and also check the /var/log/Xorg.0.log for errors.
You can test the new xorg.conf with:
~# Xorg -config ./xorg.conf
Don't forget, if there is a problem with X hanging, then cntrl-alt-F1 with take you back to a terminal,
then you can manually kill the server if you have to:
~# kill -KILL `pidof Xorg`
Sorry if you knew all this, but I'd rather you didn't have to get stuck with a hung X server and think you had to hit the power switch or something else with possibly catastrophic consquences (it ain't fun trashing your filesystem).
The cleaner the config to start with, the easier it should be troubleshoot/trackdown
issues later. By the way, be sure to save /etc/X11/xorg.conf and any other files you will be changing/replacing before messing with them, so at least you can go back to a known working configuration.