They're seeing each other, they're just not talking; the airport is designed to be used to bridge a single wired network to multiple wireless "pools" of service, which users can roam between (eg in a school). So, airports forwarding traffic to/from other airports would just duplicate packets. It's down to the level that one airport *doesn't even see* packets sent by the other (and yes, I picked the same channel, etc) - they don't appear in the packet rx counters at interface level.

There may be a way to configure this behaviour off, but I don't know what it is. I'd love to know, though!

Hugo