before you start throwing money around, did you check the Wow video settings? I would check to make sure shadows are turned completely off on each client, and that the second client is running settings on low and your main can be low-medium or whatever works for you.

also whatever program you use, check to see if you put a fps limit on it. this will help keep the second window at a certain fps so it does not boggle down your computer. I think the settings for main would be 60FPS and for slaves would be 15FPS-20FPS.