You can try this. It continues to work for us so far. Turn on first Xbox, load Destiny, log in and go to Tower. Power up 2nd Xbox or if already powered up, go into settings>all settings>network> select go offline. Now select go online and test multiplayer connection. Load Destiny, log in to character, then join fireteam. It continues to work for me so far even if you leave fireteam and rejoin. ** you probably don't even need to go to the tower on the 1st xbox (could be in orbit) but haven't tried that way yet ** Hope it helps some of you. This has been VERY frustrating to say the least!
"Local Network Fireteam Issues One of our highest priority investigations since the release of Destiny: Rise of Iron pertains to Local Network Fireteam Issues. Some players are currently unable to form Fireteams with other players connected to the same router or home network. In recent weeks, we have identified a potential cause for this issue that was introduced by a previous Destiny Update. Internal teams are actively investigating ways in which to address this issue in hopes of implementing a solution in a future Destiny Update. We would like to thank all players who have posted reports to the #Help forum concerning this issue, as it has greatly aided in our investigations. "