have you checked to see if they are both running at factory speeds? or are they both running at 8500GT clocks? of course you'll see gains with 8500GT SLI over 8600GT single card.. but you should run GPU-Z and check the gpu/mem clocks also.
replace the 8500GT with another 8600GT.. it'll work!
EDIT: Maybe the 8500GT isn't so useless after all. You could probably offload CUDA/PhysX to the 8500GT, to free up the 8600GT to render graphics only - this would amount to a "considerable" framerate difference in games that use PhysX ppu.
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