overclocking 2900 crossfire is easy. Just use Rivatuner 2.05. I thought is was difficult, but it wasn't. In the first try with Rivatuner we could get both card @ 841/952. We used GPU-Z to verify the speeds.
Foxconn has released a special tool for overclocking 2900 in crossfire, called "Quantum Force" or something.
You can get the tool and special bios here.