From what you are describing you are using winXP in dual-view mode. Due to some bug in XP it will halve your fps when you run in this manner.

I have an 8800gts with dual monitors running off 1 system. I put the monitors into one large horizontal mode (desktop stretches across both) and then run the wow's in windowed mode. Just drag the window to the 2nd monitor and resize as big as you need it. I have run 3 instances of wow and got 40-50fps on all 3 in that mode.