Play the campaign with a friend
By Benderman · retold
Find and host the community co-op campaign mod through Retold’s in-game mod browser and custom-scenario lobby.
Getting the Coop Campaign Mod
To install the Coop Campaign mod, navigate to the Mods / Browse Mods page in-game.
Then, either search for "coop campaign", or sort mods with the Campaign tag by most downloads to find the mod:
With the Coop Campaign mod selected, click the Install button.
Playing a Level in Coop
To host one of the coop campaign levels, open the Multiplayer menu in-game, then click the Host button.
When prompted, set the Game Mode to Custom Scenario and choose the desired level (all levels will begin with "coop-campaign"). Then click the Host button.
Once in the lobby, invite your friend, select a difficulty, and ready up. No other settings need to be changed.
Source: Benderman · Steam Community guide “Coop Campaign”