Did you try a system reboot and then run Heaven again.
I got strange behavior a couple times with Heaven if I alt tabbed out of it running full screen and then went back in , resumed full screen. Noticed my loading on dual gpu's was less and uneven. This is in a Nvidia SLI setup.
I keep a OSD of the two gpu's and their load #'s in the corner of the screen.
Anyways, by happen chance, it was about 10% less performance.
I was definitely getting the benefit of both gpu's VS just 1, but it was 'off'
Thats not something I would notice in gaming obviously, maybe in another timed/scored benchmark.