I think the CPU in their set up is basically a smart data switch. There is a way to sync up the GPUs, (SLI ??) but I've never had more than one of these cards in my hands!
This GPU is amazing - it can do over 12,000 threads at once --- and that's just one card. Realistic however - according to the docs I've looked at - is about 5,000 threads. The system mentioned above has 4 cards = 20,000 to 48,000 threads all at the same time. (And they can avoid thread lock!)
Holy Rendering, Batman!