You can either Extend or Stretch and to be honest it's been so long since I screwed around with an ATI card I can't remember which one works for 3D apps. But I do remember the key was to do it through the CCC and not through the Windows display properties.

First lets reset it back to single monitor mode. Right click your desktop and go to Properties. Select your secondary monitor and uncheck the "Extend my Windows desktop onto this monitor" box. Click Apply, OK. Then reboot. When your machine restarts it should only display on the main monitor.

Now open up the Catalyst Control Center and select Displays Manager. You should see your main monitor there and your secondary in the "Attached displays currently disabled" box. If not then... screw around until you have the secondary disabled. Now, right click on the disabled monitor and you should some options to "Extend" or "Stretch". EDIT: I dug up an old post of mine ^.^ what you want is Stretch mode. So enable that and then try. Let us know how it goes.