Add this to your XF86Config-4
Code:
Option "TwinView" "1"
Option "SecondMonitorHorizSync" "30-50"
Option "SecondMonitorVertRefresh" "60"
Option "MetaModes" "800x600, 800x600"
Option "TVStandard" "PAL-B"
Option "TwinViewOrientation" "Clone"
Note these are the config values for Spain read Nvidia README for correct value for your geografical location.