A couple things to note. Crossfire does not function when in windowed mode. Your tests indicate you are running in windowed mode unless that was a log over time while in fullscreen. Another possible issue is that at 720p, it's quite possible you will cap your performance already and adding another GPU may not give any performance increase as a result of the CPU holding it back. This is common on low resolutions with powerful cards.
While this may not have been your issue, more powerful cards which are bottlenecked by the CPU, will not give more FPS. This is because the CPU is holding back performance. This happens most often with low resolutions, like yours.