I'll hop on this too.
I have a 4850 and a 7850.
I wish to use my 4850 for games/desktop use.
And the 7850 for mining.
I've already tried it and I get a hyper transport sync flood error once I start mining on the 7850. Not sure what to do..?
Did you get any joy on this? I've picked up a 4770 as a 2nd card to go with my 7850 and would love to get them both working together.