There is nothing (for the most part) built into Wowza to communicate externally. You would do it the same way you would using any other two Java programs.
You could use:
1. Java sockets
2. HTTP Provider
4. You could have your flash client connect to a stream on each server and use Flash RPC. (but that's a little weird)
Normally you would use a Shared object to let flash clients communicate with each other, for example for Chat. You could potentially use one of the above external methods two sync the shared objects on each server....