It seems Can more than one ic880a gateway run behind a typical home firewall? says all is fine:
And UDP ports and firewalling explains:
So, it seems you don’t need to (or: should not) set up any port forwarding at all; the Semtech UDP protocol should already fix this by opening connections, assuming these do not time out
(and do not happen to choose the same random port numbers…) and then NAT will just take care of everything.
And indeed, I’ve been using two gateways using a FRITZ!Box 7360, without configuring anything (only one was receiving commands from TTN, but that was handled by the router automatically).
If UDP does not work for you, then maybe using the “new” TTN packet forwarder protocol, which is using TCP/IP, might fix this. (Even when using that on both gateways.)
Note that TTN will select only one gateway for downlinks. It might also detect single-channel gateways that do not support downlinks, and then discard those.