I have the impression to ask something stupid, but i bought two versions of the game on steam for me and a friend. We can play normaly, bu we do not know how to play together... Need help please
... or either of you can invite the other into an existing game and have them control whichever characters you want (via the Assign Characters screen which you get when you hit the Escape key after they join your game).
What you can't do is move a character from one solo campaign into your friend's solo campaign.