From Twitter user nobleactual
glitch into the co-op:
In order to get this working, Halo Infinite must be offline, then, you need to connect a second controller and sign into an Xbox account. After that, start the campaign with controller 1 and in-game hit start and then back. On the second controller, hit start to add the second player to the fireteam.
This can bug your Halo game saves so head’s up if you’re going to try this.