Comment 7 for bug 726638

I revisited this issue recently, and discovered that I have to use the nvidia-settings program to get dual screen going with the proprietary display driver, since xrandr doesn't work. So, once I discovered that, I simply configured my 2nd display as a "Separate X Screen" (in the "Configure" popup) and chose "Enable Xinerama". Then dual monitors is finally working for me with the nvidia graphics card (and best drivers) enabled.