Multiple XBox Ones, only one can play Multiplayer

We have 2 XBox Ones. Only one of us can play TF2 Multiplayer at a time, the XBox that starts the game first "wins" and can play multiplayer.

Steps to reproduce:

Have more than 1 XBox One on the same LAN behind the same Internet Gateway and External IP.

Start Titanfall on XBox A, then start on XBox B. 

XBox B will sit at Connecting to Data Centers.

Exit game on both consoles.

Start TF2 on XBox B, then start it on XBox A.

XBox B will get into Muiltiplayer, XBox A is stuck at Connecting to Data Centers.

Attempted fixes:

Full reboot of everything including router and xboxes.

Flush UPnP sessions on firewall.

Can reproduce 100% of the time.

I also noticed that TF2 doesn't appear to open additional ports via UPnP, only the XBoxes Teredo connections are opened (3074 for first xbox to boot, then random high for the subsequent ones)

Assumed root cause:

It appears that when a player connects to TitanFall 2 data centers, it registers the external IP address as 1 player.

Then, when another attempts to connect with the same external IP, it won't allow it because the external IP is already registered and connected.

This would explain why whoever starts TF2 first, can connect and subsequent XBoxes cannot.

Also, explains why you can "flip it" by shutting down the games and starting them in the opposite order and it works fine.

How Respawn/EA can fix this:

IPv4: Use UPnP to assign needed ports and talk over those ports, ignore external IPs.

IPv6: Preferentially Use it.