Première chose as tu installé les drivers livna pour les drivers nvidia.
Secondo tu dois configurer ton /etc/X11/xorg.conf dans la section screen et activer le twinview:
Section "Screen"
Identifier "Screen0"
Device "Videocard0"
Monitor "Monitor0"
DefaultDepth 24
Option "TwinView" "True"
Option "TwinViewOrientation" "RightOf"
Option "UseEdidFreqs" "True"
Option "Metamodes" "1280x1024,1280x1024; 1024x768,1024x768; 800x600,800x600; 1280x1024,NULL; 1024x768,NULL; 800x600,NULL"
Option "SecondMonitorHorizSync" "27-92"
Option "SecondMonitorVertRefresh" "50-160"
Option "RenderAccel" "true"
SubSection "Display"
Viewport 0 0
Depth 24
Modes "1280x1024"
EndSubSection
EndSection