The problem with a co-op mode is how it will effect the final game. Larian would either have to do two different versions of the game (which they don't have the resources for), or make restrictions to the game so that it will work as either a single player or multi-player game.
I suppose the easiest way to allow for multi-player without compromising the story or quest depth would be to have one person playing the main character, and any other players as secondary party members who can not get quests, etc. Alternately, any party member could be allowed to get quests, but they would effect everyone at once (ie once a conversation was started, all players would see that until it was finished, so there would be less chance of conflicts).
Either way it would add complexity to the game, for a feature that may or may not be well received by most people in the form it would have to be implemented in.
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