https://www.reddit.com/r/halo/comments/rhzuh5/someone_in_a_facebook_group_managed_to_glitch/
They’re currently investigating how to replicate it. Appears to be a console-only thing due to split-screen.
EDIT: Source from noble_actual_yt
I got it to enable splitscreen! Sort of…
Infinite must be offline. **Note: this can’t be true offline (disconnecting from the network) from my current testing. I believe It needs to be that weird stage where Infinite disconnects you after quick resuming.**
Sign into a second Xbox live account with your second controller
Load halo infinite campaign
Press start on main controller
Hit back button
Press start or back on second controller to join fireteam
Co-op enabled!
Some notes: 2nd player has no HUD. FOB’s are completely busted, marines no longer spawn at FOB’s (assuming it’s because for P2 the FOB is not captured) . P2 does have their own tacmap and upgrade tree. Game will soft lock if either player dies. P2 will fast travel with P1.
I think the issues stem from the players progress being out of sync. An interesting test would be a new game from the beginning to see if that fixes the FOBs at least.
Edit: after softlock i can’t seem to get it working again. Very finicky.