if your using anything to launch your wow's keyclone maximer/is screen layout etc. it is not in fullscreen but windowed mode, with ati series cards 3d acceleration does not automaticaly update in windowed games. You can use 3rd party apps besides ATT such as riva tuner etc to create profiles for your games etc and set up clock speeds based on applications running i.e. wow.exe, which will take your card out of 2d clocks when launched. If you still want to use your secondary car IS has a built in workaround for it, wher eyou build the profile and assign the gpu for it, if you choose not to use IS you can still do it the old fashioned way

1. disable the primary gpu, launch your lower wow
2. enable primary gpu and launch primary wow
a real PITA to be honest with you so using IS is the best bet if your gonna do it.