Step 1: Download EnvyNG
sudo aptitude install envyng-core envyng-gtk
Step 2: Use EnvyNG, from Applications -> System Tools -> EnvyNG, then download appropriate drivers, this may take a while. restart gdm once installed.
Step 3: from System -> Administration -> NVIDIA X Server Settings
Enable TwinView, and click “Save the X Configuration File” , and click save.
Reboot
done!