Ya'll stupid as hell.
OP brought up a real issue though. Since this is a Sony dominated forum the set up was too easy but in all seriousness, I've had the same issue.
When you play backwards compatible games on XB1 logging in to the second player can be cumbersome. The XBOX guide button sends you back to the XB1 menu, instead of the 360 menu, which makes it a pain to sign in to other player's profiles. It's not always an issue, but it has happened to me a lot, so much so that I stopped worrying about playing co-op 360 games on XB1. In the case of Gears of War, the second player didn't sign in before the game was launched. Somehow, this corrupted my save and causes the game to crash immediately when starting it up. The crazy thing is, it happened with every gears of war title, and now I can't play any of them on my XB1. I uninstalled the games, deleted the saves, and still no luck. I checked online and apparently this is a known issue.
Gears of War 2 crashes on log in | Technical Support and Help | Forums | Gears of War - Official Site
https://forums.xbox.com/en-US/thread/36E2FF25-518D-45EE-A62A-8B76B99DAC64
Apparently, if you don't sign in both controllers BEFORE launching the game, when you go back to play, the game breaks, and somehow it's link to your gamertag permanently.
I've tried everything to get the games to work again with no luck. I pretty much cannot play the OG gears of war games on my XB1 ever again under my gamertag.