This is partly a driver issue and partly a hardware framework issue. The BEST option is to span ONE screen, ONE port. In order to do that you need a BIG screen or small WoWs. Once you hit the other PORT of the same graphics card - fps start to go to crap.

You can try installing another video card although MOST motherboards only have 1x x16 electrical slot. Some have 2x x8 PCI-E ones, which may work with little loss until you go up to an 8800 GTX or so. Then the bandwidth limitation starts to kick in.

My $300 motherboard has 2x x16 electrical slots and a third x8 slot... but it still seems cheaper and easier to me to simply buy another computer rather than work it all out in software and use small WoWs, multiple video cards (which still has some non memory card bottlenecks) and / or huge 30" screens. You can get a computer that plays WoW at 30+ FPS all of the time for $400 - $500.