Do you have UPnP set up? If not, that's why you are getting it; simply put, one Xbox uses one the network ports needed by Destiny 2 making it unavailable for the other Xbox.
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Having port forwarding or a DMZ configured at the same time as UPnP will cause conflicts; only one of those solutions should be used at any given time. [url=https://help.bungie.net/hc/en-us/articles/360049496531]Network Troubleshooting Guide[/url]