Parece que esse é um problema conhecido. Isto é o que eu encontrei nos Fóruns do Ubuntu . Espero que ajude você.
Re: 11.10 dual monitor nvidia card problem
Hi folks! I guess I've found a solution for the multiple displays problem... only thanks to you guys.
1.) disable active drivers (if you have any after fresh install f.e.)
2.) install nvidia drivers current (not pre-release)
3.) reboot
4.) type cd /etc/X11/ and then cp xorg.conf xorg.conf.backup (it makes copy of your configuration file... {just in case})
5.) open up the terminal and enter: sudo apt-get install gtk2-engines-pixbuf
6.) after installation enter sudo nvidia-settings (it opens up the Nvidia X Server settings)
7.) goto X Server Display Configuration
8.) set your settings, BUT! don't hit apply... when you are finished hit Save to X Configuration File
9.) log off and log in again
and voala it should be done now! I had problems saving my X configuration file but I fixed it by the step number 4