Excuse my ignrance here.
These are two different video cards?
Have you tried the "AMD Catakyst control center" ?
Generally speaking, there seems to be two methods of multiple monitors, xorg.conf, or xrandr.
You should be able to find plenty of examples of dual monitor xorg.conf around by web searching. the only problem I've ever had is to have to occassionally go back a bit and give full specs of an older monitor.
You might have to pop a terminal to run xrandr which will end up being ust a one line configurator. the catch for my card was that the two displays were called DF6 & DFP7, so the invokder was something like(vague memory)
xrandr -dsiplay(?) DFP7 right-of -display DFP6.