XCOM 2 fans at Dragonpunk team have released a co-op mod, which you can get at Steam Workshop. The Tactical Co-Op modification introduces cooperative gameplay for two players. Playing this sci-fi turn-based strategy together with a friend or a complete stranger may prove an interesting challenge, and the mod creators are waiting for Firaxis Games’ approval in order to implement 12-player co-op. Note that this is still beta, so it may contain some bugs and does not support turn timer missions and Ironman mode.
Dragonpunk have also released a couple of other XCOM 2 modifications, which you can view here. The latest installment in the popular turn-based strategy games series launched early this year to a critical acclaim. The developers at Firaxis Games provided extensive support for modders, which resulted in the abundance of great game modes.