Nvidia has just made everything a tonne easier for configuring dual monitor, colour, opengl and antialiasing settings with its Linux version of a graphics control panel.
As always you should back up your xorg.conf file before playing with it with a simple sudo cp /etc/X11/xorg.conf /etc/X11/xorg.conf.backup in terminal.
To access the control panel it is as simple as gksudo nvidia-settings and configure away. Once finished configuring with dual monitors, click the save to ‘x configuration file’ button and click apply then quit. You will proberbly find you will need to reboot GDM + X11 by pressing ‘CTRL + ALT + DEL’.
Good luck and have fun.