Tienes que modificar el /etc/X11/xorg.conf, el mio lo tengo asi:
Section "Device"
Identifier "NVIDIA Corporation NV34 [GeForce FX 5200]"
Driver "nvidia"
BusID "PCI:1:0:0"
Option "DPMS"
Option "TwinView" "1"
Option "ConnectedMonitor" "CRT,TV"
Option "MetaModes" "1024x768, 1024x768; 800x600, 800x600; 640x480, 640x480;"
Option "SecondMonitorHorizSync" "30-50"
Option "SecondMonitorVertRefresh" "60"
Option "TVStandard" "PAL-B"
Option "TwinViewOrientation" "Clone"
Option "ConnectedMonitor" "CRT,TV"
Option "DigitalVibrance" "0"
EndSection
Un saludo!