XCOM 2 is a stressful game. Whether you're facing down a horde of aliens or trying to reach one of your soldiers before they bleed out, it will test your mental resolve at every step. What better way to ease your burden, then, than by having a friend join your campaign to offer the emotional support you need (or laugh at you when you miss a 90% shot, either way.) Team Dragonpunk, the modders developing the ambitious total conversion for XCOM 2 - Dragonpunk, have been thinking along similar lines and have taken some time out from animating cyber-punk dragons to bring players the first co-op mod for Firaxis' strategy game.
In its current form, the mod allows two players to connect over a network to play campaign missions with each player taking control of a portion of the squad. As you can see in the video below, players can invite co-op partners from the squad select screen prior to starting a mission.
The XCOM 2 mod, called 'Tactical co-op' is currently in its Beta phase and is not compatible with missions that feature a turn timer, as it was built to work with the Guerilla War: Recovery turn system mod, and does not work with Ironman mode due to the save system. There is also an alternate version of the same code that exists as a stand-alone mod called 'Dragonpunk: Soldiers of Fortune.' Both pages state that the modders are currently waiting on approval from Firaxis for code enabling up to twelve player co-op.
For now, at least, players must both be in the squad select screen and one player must invite the other to allow them to join a co-op game. Both players must have the mod installed and should probably try running with the same set of mods installed in general, or disabling other mods altogether. Team Dragonpunk advises they are not yet aware of how the mod will interact with other 3rd party additions. You can find full instructions on how to use the co-op mod here.
For more information on Team Dragonpunk and their planned total-conversion mod for XCOM 2 that will add magic, dragons, stealth, and more, check out the team's official site. If you're suitably impressed by that, you can opt to fund the project through its gofundme page. You can also read the emotional story of how creator Dan Connery intends to create the mod as a springboard to a full game, in tribute to his father - the recent victim of a tragic fatal shooting, on the funding page.