Airplay to more than one screen concurrently?
I just tried it with my mid-2011 Mac mini running 10.8.  I can choose to mirror to either of my ATV's, but not both at the same time.  One ATV is version 2, one is version 3, which could have some effect (they are different native resolutions), but I somehow doubt that's a factor.  I'd think multiple mirroring could create quite a demand on the GPU and the network, so I don't think it's supported.