Lucky Day, I think he is asking for NWN2 not the first NWN.
Anyways, from what I have seen it plays just fine. While both players can see the dialogue, only one of you will be actually selecting the choices and of course the story wont 'notice' the other guy
Heres few videos on youtube of 2 players playing CO OP NWN2 main campaign
http://www.youtube.com/watch?v=8mBED…el_video_title