You might try turning on the CrossfireX Status Icon to see if it's being used. This will display an ATI logo in the upper right corner of the screen when CrossfireX is running.
Right-click the tray icon and select your primary adapter, "CrossfireX Settings", and "Show CrossfireX Status Icon (where applicable)"
I don't have WoW, so I don't know. Use the above instructions to find out.