Um.... Well this is what I do...
Go into system, and then the nvidia app. (u should have it if you have updated to the latest drivers) When you go into the nvidia app go to x configuration and then you will see your display and the current resolution.
Anyway, look under that and you will see detect displays. Press that and it will detect your second display. Click the second display and then you will get an option as to either xtend the desktop and twinview (or something like that). Pick what you want, apply it and you should be good. I'd do screenies but i'm not on my pc
edit: and in case it doesnt apply you may need to restart x... i think the command is ctrl+alt+f1?... dont quote me on the command.
Laptop: HP DV6700t - Core 2 Duo T9300 2.5Ghz, 3GB RAM, Nvidia 8400m GS, 250GB HDD. Ubuntu 12.04 and Windows 7
Phone: Samsung Galaxy Nexus