Crossfire will only double the speed of your slowest card and only If they are the same family (ex. 5850 and a 5870)
Plus when in crossfire mode, work gets sent to the 2nd GPU via the 1st GPU which is the cause of the bottleneck.
Crossfire is a real bad idea for mining.
You're better off mining on each card separately, especially if you have different GPU's
While this may apply to some games and things, I'm fairly sure it doesn't apply in the same manner to bitcoin mining. Bitcoin mining uses special kernels that are executed on each GPU regardless of whether Crossfire is enabled or not. From my experience, Crossfire causes neither a loss nor a gain in speed, but
it does result in much higher CPU usage when enabled, which makes it a power hog.