This is my story using Crossfire Xtension for Assassin Creed 2, the game by default doesn't support Crossfire. But at least it can be forced with Crossfire using the ATi CF Xtension, it allowed me to force Crossfire support with Assassin Creed 2 which doesn't have good Crossfire scaling. The AFR Method didn't work reliabily, The Scissor Mode showed much better results with great minimum frame rate, but never exceeded the 60% GPU usage in each individual GPU even with 24x Edge Detect Anti Aliasing, and Supertiling maxed my GPU usage showing more consistent frame rate than Scissor Mode aka Split Frame Rendering, specially in minimum frame rate. Assassin Creed 2 is a severely CPU bound game with a sickening Frame Rate cap of 62fps, awful with a 75Hz refresh rate.
Assassin Creed 2 was so CPU bound that I couldn't find a huge improvement in frame rate. Using 24x Edge Detect Anti Aliasing and using Adaptive Super Sampling at 1280x1024, the game felt choppy with slowdowns dipping in mid 20's at the minimum frame rate with an average of 33fps, going to Crossfire, allowed me to have a minimum frame rate of 33fps, average of 54fps and a maximum of 62fps which is the limit of the frame rate cap of the game. The performance difference between Super Tiling and Scissor Mode was very hard to notice, Super Tiling offers you slighly higher minimum frame rate at the expense of huge GPU usage, near 99% on each GPU, compared to the 60% average using Scissor Mode. So Super Tiling is less efficient in terms of power/performance ratio.