There is no net benefit. APUs are limited to a much, much slower memory interface as well as memory technology compared to discrete cards, which is why the latest desktop APUs (not counting the Devil's Canyon marriage) are only able to pull out performance figures equal to the lowest end of discrete cards. Combine this with the non linear scaling of multiple GPUs, and the fact not all games support multiple GPUs, and you end up with a much larger headache than it's worth to add, literally, a couple of frames per second.