It looks like a single 6950 takes a 7% hit going from x8 to x4. In Crossfire, this effect will be magnified because each GPU gets the same data and since one GPU is only getting it at x4, the other will only be getting data at x4 as well.
I imagine that this will very negatively affect the benefit of adding that 2nd card although I'm not totally sure.
EDIT: I looked on the 2nd to last page of that link I posted and they did the exact thing you are talking about. Looks like an average of -10% performance penalty for using x4 with a worse case of -24%.