low FPS, that is very strange. I don't seem to have a problem getting 60 fps even with two of them running on my Nvidia 8800 machine or the the radion x1800 machine.
The machine that does get crippled is my Dell laptop with a Nvidia 7950 in it. multiple copies of wow seriously cuts the FPS on it.

were you running either of these under keyclone, I know the maximizer has a setting to limit the max fps, i set that number to 60 and I usually get 60 fps out of two copies of wow ( assuming i'm not in a busy major city like shat )