Team Dragonpunk has released a new mod for XCOM 2 that adds tactical co-op support to it. This current version of this mod supports two players, however the team is waiting on Firaxis to approve code for 12 player co-op.
In order to play this mod, both you and your co-op buddy will have to install this mod. Then, both of you will have to load a save (any save will do the trick) and head over at Squad Select screen.
Afterwards, the one who will be hosting the game will have to load up a mission and get to the Squad Select menu. He’ll then have to set up the team accordingly and then remove 1 of the soliders. A new button under “Select Soldier” named “Invite Friend” will appear. After clicking on this button you should have the Invite Friend Steam Overlay popup.
Invite your friend, click the return to game button at the bottom right, and click accept.
Those interested can download the mod from here.
Have fun everyone!