I use my machine for a lot of things, mining is one of them. Whilst i'm not at the PC i put the intensity to max.
My main reason for taking out the 6850 is I only get 200-218KHs out of it and when mining with the pair both monitors stopped displaying. I jumped to the conclusion this was related to power and after a few minutes i turned off the PC removed the card.
I'm tempted to get another 7950 but will wait till the 9 series comes out as it should either price drop the 7950 or the new ones will be worth buying.
In theory i can probably run both cards ok but i didn't think it's worth the risk.
RChevalier point about the PSU is a good one and makes me think about getting a better rated PSU as mine is only a bronze. My old PSU will be used for a mining rig with the 5850 and the 6850 in it.