The only thing I have heard about this is way back someone crossfired a 3870 and a 4850. The performance was less than impressive, I would call it bad actually.
kppanic:
it does not work that way, its symmetric multi-processing, just like SLI, Multi-Chrome, etc, it requires the processors to carry equal portions of the compute load, which is why you cannot add a 4350 to a 4870 and gain performance