I had the same issue Frostie where it would only open one when I went back to the original. You can easily fix this by going back into the configuration and putting -d3d9 in the main executable parameters.

Lax: I did experience an error while running this on my system 9600GT SLI. It loaded the clients just fine, but then when I switch from one wow window to another it would lock up and the clients would be black windows. I am using two separate profiles since I have the main screen rendered by one card and the remaining 4 clients rendered by the second. I'm not sure if that had anything to do with the error.