there's a risky possibility to enable dualhead (when manually editing and the system-config-display doesn't work) :
1.) delete xorg.conf
2.) switch to runlevel 3.
3.) login as root and enter "system-config-display", this will autodetect your videocards and you have a dualscreen.
4.) customize your xorg.conf (e.g. nvidia instead nv and so on)
for me this worked but i take no responsibility for that :-)