depending on the kinds of NAT your routers have, and the kinds of local firewall (if any) the computers have, that should work.
you will maximize your chances of a DIRECT_CONNECTIONS connection working between the computers behind the *same* NAT if both ends attempt to connect to the other simultaneously.
for connecting computers between the two NATs, it will depend on whether they are symmetric NAT and/or port-restricted cone NAT. for more information on that, please see Matthew Kaufman's posting:
Thank you very much for the answer!
1) So I assume if all users are behind cone NAT (2 behind a cone NAT and another 2 are behind another cone NAT) - then they will successfully communicate to each other (video & sound) every time (if there is no firewall)?
Am I correct?
2) If one user has a firewall on his PC - then what UDP port numbers must be opened?
1) those cases should work.
2) RTMFP uses random ports, and of course NATs would randomize the ports even if RTMFP used fixed ports. a firewall should be set to allow "outbound UDP" (which should also mean return/responses), and then RTMFP P2P should work.