I don't feel so bad now. A friend lent me his 5770, and when in Crossfire mode, Catalyst Control Center doesn't report anything for MHz or temperature for the second card. The first card is reported fine.

RealTemp uses the CCC libraries to get temperature information so I'll see if I can find a way to get data from both GPUs even when one core is having a snooze. CrossFire support is a work in progress.