From my understanding, monitors attached to any card other than the primary (crossfire or not) will only run in software mode without extensive juggling of primary displays each time you start up the programs. The video card flexibility is nowhere near what you can do with a CPU. Ultimately, when it comes to performance you wouldn't see a gain with a 2nd video card even if WoW could take care of it - you would already be maxed out on one card that fast.

Buy two cards! Many, many, modern games utilize them. WoW is more of an exception.