WoW doesn't support crossfire or SLI. In fact, most MMOs don't support it.
The code just is not optimized for it and as a result, things actually run worse. Even if they took the time to support Crossfire and SLI, I don't believe performance would increase much.
WoW is old game and not graphic intensive, but rather CPU and memory intensive due to its constant expansion of code,players...
@Doomsknight : Simply put wow models are just too low on polygons(detail) and they can run perfectly fine on old graphic cards,increasing performance from 50FPS to 150FPS is only statistic you won't notice much past 25FPS in 3rd person online game if they want to use the gained power they need new models for game and that is making completely new game...which they are doing with titan.And only thing why game is CPU intensive is because they have to keep their subscribers interested in game by adding more and more content without actually making a new game.