To use both GPUs of HD 5970 under FurMark:
- run the etqw.exe version of FurMark
- run FurMark in FULLSCREEN
For you information, I get a score of 5420 points with my HD 5870 in 1920x1080 fullscreen, no AA, 60sec.
With your HD 5970, you should be around 7000 / 9000 points in 1920x1080.