Keyclone has this feature built into it. However I noticed that after some time playing (20 minutes or so) with a fair amount of switching between windows, all of my WoW instances would become FPS capped at the value I set for maxfpsbk (12, which is unplayable for the one you're focused on). This was on my Vista machine, so it could be an issue there since I don't think Keyclone officially supports Vista. I ultimately ended up turning it off, the extra click doesn't bother me too much.