I have a 5870+5850 connected with a CrossFireX cable. Everything works fine under Linux where crossfire is not enabled (and not supported ?).
Instead, I have a problem under Windows 7 64bit: the 5850 reports to have 20 compute units (it has 18) and produces incorrect results when used with OpenCL.
I assume the problem is related to the CrossFireX connection because I had a 5870+5770 installed on the same PC last week (not connected with the crossfire cable) and they were working fine under Windows too. Or my be it is just the 5850 to be detected as a 5870 (i.e. with 20 SIMDs).
Anyway it looks like a driver bug (I use Catalyst 10.2).