Your having the problem for the same reason people that have 3 video cards have - WoW very strongly likes to use the primariy video card to do all the processing and its difficult to trick it into using the others.

Sure you might have 2x or 3x cards but if check their load, all the 3D side of the crunching its being done by one GPU only.

For that exactly reason I bought a ATI 5870 over a GTX 295. I really wanted more Video RAM but it wasn't worth it for the dual-gpu issues.