Only considering the AP, then there is no way to allow two different SSIDs to talk to each other. However an AP does need to plug into something to be useful, such as a switch, and that can provide the capability.
Well, technically SSID's themselves don't communicate with each other, but you could have more than one SSID in the same VLAN and subnet or you can route between them via a layer 3 device.
As @BrandonS said; pretend the two SSID's are separate Ethernet segments. Weather they can talk to each other is just the same as in a wired world. If you provide a method for them to talk to each other then they can - otherwise no.
Thanks! Yes, I understand if you have route between SSIDs then you can communicate. Sorry I wasn't clear. I mean in general, on the AP itself, w/o any routing, the SSIDs wouldn't be allowed to talk.
Only considering the AP, then there is no way to allow two different SSIDs to talk to each other. However an AP does need to plug into something to be useful, such as a switch, and that can provide the capability.
