General answer first, then specific (but vague ) :
Under the current RandR model you are practically limited to a single GPU. The addition of multiple GPU support to RandR depends on implementing a mechanism to split drawing operations across multiple GPUs, commonly known as "shatter". MostAwesomeDude will be working on shatter over the summer under a GSOC project, so if nothing else by the end of the summer there will probably be a working prototype and any remaining issues should be a lot more clear.
For the specific case of multiseat you probably don't need shatter and just need a way to configure each of the GPUs, but I suspect that adding "multiseat only" multi-GPU support to RandR is considered a lower priority than a more general solution.
It's possible that the current drivers (radeon running in "zaphod mode") might be useable for multiseat today; I saw a few posts that "implied" it could work with multiple GPUs but did not actually say it ;(