Rather than using the system-config-video app, manually configure dual
head through the Xorg config file. Everytime I have dealt with the
gui's I haven't been able to get dual head to work at all.
this should help you out a
greal deal, though you'll need to use lspci to determine the BusID for
each card (it'll be different since you aren't doing dual head from one
Shoot me an email if you're still having trouble,