If your network setup only exists out of a gateway (modem with built in router) then you can assign one console as DMZ and port forward the other.
Since most routers don't support double NAT this will be the only way to get an open NAT on both of them.
But could you fill out the following information to I have some more insight to what your setup is and how the issue might be resolved.
What Country/State/Province do you live in? Modem brand & model number: Router brand & model number: How many Xbox 360 and Xbox One consoles are connected to this router?: Wired or Wireless (for each console): Things you have tried: Is UPnP Enabled? Yes/No/Unknown: Do you have a NAT error? If so, what is your NAT type?: Any Error Codes you encounter: Who is your ISP? (Internet Service Provider):
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