I'm running Ubuntu 12.04 64-bit, Precise Pangolin, with a PNY GTS 250 1GB video card and a monitor plugged into each of the DVI ports. I'm using the proprietary drivers (post-release updates).
If I set anything to do with Separate X Screens up in nvidia-settings (and write it to xorg.conf and reboot), my second monitor has a grey background, no menu bar, no ability to have a window on it, the second monitor doesn't get picked up in a screneshot, and if I move my mouse cursor to it it's an ugly black X. Plus, my network is unable to connect to anything.
If I subsequently delete /etc/X11/xorg.conf and reboot, everything goes back to working, albeit with a single monitor activated.
If I set anything to do with TwinView up in nvidia-settings, my second monitor starts working, but it isn't seen as a second monitor by Ubuntu, so I can't apply color calibration to it separately. Plus, my mouse gets "caught" between the monitors every time I try to move my cursor between the two.
What gives? If it helps, this is the xorg.conf that nvidia-settings generates for Separate X Screens.
Best Answer
After such a struggle with settings here and settings this with the Nvidia graphics GUI I clicked on dash, typed in driver, selected additional driver. Here you should see 2 options after opening addition drivers
NVIDIA accelerated graphics dirver (version current)[Recommended] NVIDIA accelerated graphics dirver (post-release updates)(version current-updates)
I had 1 selected as this is how I installed my Nvidia driver initially (Had trouble downloading directly from the site and installing it). This then broke and after a struggle as mentioned above I selected 2 and clicked on activate, restarted my machine and boom. Up and working np. Here is my xorg.conf file output
Edit: Oh and how to get rid of the mouse getting stuck between the 2 screens: