If they both get the green light, why not have co-op in one of them.

Personally I am not a big fan of multi-player games (can be fun, but I don't know anyone outside of this forum with similar tastes in games, or anyone that would match my play schedule for very long), so would prefer Larian invest their resources in other aspects of the game(s). To do co-op right they would basically then be making a third game, or the design of the game with co-op would be limited to things that would work for single player and co-op (which could be the intent, anyway).
Early in development D2:ED had a multi-player component, but that was dropped when it put to many restrictions on quest design, etc.