Quote Originally Posted by Psychophoria View Post
I have significantly reduce the graphics quality of both clients, and it seems to of alleviated the problem.. to an extent. I notice that the second client is still around 10FPS or so while not in focus.
I feel that with my computer stats, I should have no problem running two clients at decent graphical settings.
I think it must be some strange setting in my PC which is limiting background programs from running at their full potential.
One thing you may want to do is try from a fresh and minimal install of Windows with just the updated graphic drivers and only WoW and retest.

You can do this semi-non-invasively by adding a new boot device and install and test with it or maybe via setting up a dual boot (though I haven't setup dual-boot of 2 versions of windows).

You could also try a full reset of your current windows install or re-install over your current setup and test and then restore your data though that is more invasive.