I run all 3 of my 5800 cards with crossfire enabled, and CGMiner adjusts the clocks, mem, and VDDC independently. Is this different for 6XXX series cards?
I find I can't independently adjust the mem clocks down to 200MHz with CF enabled. It's notoriously difficult to break the link between core and memory speed on the 6xxx series cards though. It might be possible to clock each card independently and get mem clocks low with CF enabled, but I can't seem to get it to work.