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At this time there are no plans to implement any sort multiplayer into the game as Divinity 2 is a rich single player experience and adding Co-op to that would sacrafice story depth.

A quote from Swen Vincke CEO Of Larian.
"We actually supported coop multiplayer for quite some time but had to sacrifice too much story depth to maintain it, so we put it on hold end of last year. It turned out that we would've had to make two games if we wanted to have a good single player experience and a good coop multiplayer experience, and that just wasn't doable with our team size."