I think you may not be receiving many replies because cooperation is something that has already been implemented and stressed in the original NWN.
You are totally right, it has, undoubtedly, made the game much more fun for both me and thousands of others.
Because of this fact, I find it extremely hard to believe that Obsidian would remove this mechanic, as it would negate a rather large portion of the game. It would taking a huge step backwards.
Based on this, we can probably be around 98% certain that co-op will be a well-supported feature in NWN2.