There's a way to enable couch co-op in Halo Infinite, but it would be wise to wait.
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For most fans, the fact that Halo Infinite released without any sort of campaign co-op was quite disappointing. 343 Industries did say that co-op would be coming alongside Season 2 of the game, which may not be released until at least May 2022.

However, there is a way to "play" Halo Infinite with local co-op thanks to a little workaround. This was discovered and posted to the Internet by nobleactual on Twitter. A video showing off how to accomplish this can be found below.

You first need to set your Halo Infinite status to offline. From there, connect a second controller and sign into an Xbox account. Next, start a campaign with the first controller. Once in the game, hit Start and then back out. Then hit Start with the second controller and add yourself to the fireteam.

As this is not at all an official way to play co-op, there are some major drawbacks to using this method. Issues encountered when trying this method include the total lack of a HUD for the second player. The lighting also seems to be completely broken. You won't actually be able to progress the campaign, nor will you be able to use any of the FOBs (Forward Operating Bases). Oh, and if one of the players die the game will freeze and force you back to the main menu.

Perhaps the biggest drawback to this whole thing is that it can and will completely ruin your existing saves. They will no longer work after trying this. Even if the other caveats didn't scare you away from this, having your saves become unusable probably will.

It's a neat trick, but it's honestly in your best interests if you just wait the several months for 343 to officially add co-op to Halo Infinite.