The Apple AirPort routers were not originally listed on the Microsoft Xbox site as supported devices for Xbox Live! So it would be "hit or miss" as to whether or not you can get an "Open NAT" for a single Xbox let alone two.
Most that have been successful either configure the AirPort for port mapping or place the Xbox into a DMZ. Apple calls the latter a Default Host.
I would recommend that you go to the Xbox support site and check up on their current list of supported routers to see which are Xbox Live! "Open NAT" compatible.