Perhaps it's a limitation with your video card. What rez are you running? Have you tried to back down both instances to a smaller rez? I have the GTX 260 card and can run up to 4 instances comfortably on the same machine without frame rate issues @ 1920x1080. I boot the 5th and I start to get frame stutter. Also, if reducing the rez doesn't help, perhaps try a different client (SoF or Titanium).
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