I have an HD5970 with two internal GPUs and in windows 8 with catalyst 12.8, 12.10 or 12.11 (beta4) the second GPU is never used. Always at 0% usage. I used GPU-z to confirm that the 2 GPUs were enabled and they realy are, but for some unknown reason, 2nd GPU is never used. In windows 7, with 12.8, 12.10 or 12.11 drivers everything works perfect, with 2nd GPU working. I tried this with 3dmark 06, 3dmark Vantage, 3dmark 11, Crysis 2 and FIFA 13. One strange thing is that in windows 8 appears a new option in Catalyst (that doesnt exists in windows 7) to enable crossfire. I think the problem is here. Windows 8 or the Catalyst drivers are seen only one GPU and not a one card with two GPUs.
I would appreciate any help. I've tried everything with no sucess.