Most impotantly, how exactly are dialogs and quests handled.

Does the coop partner take up a follower slot?

I don't know actually...as I never played the OC's.

I've heard that it should be kind of ok though. Sometimes you even see some on the internet you can join.

I don't know if the whole campaign is possible to play in MP or parts of it...

There might be some fixes for MP on the nwvault site....

...or ask in the Bioware Social forums...there is always a wizkid there... :-)

http://social.bioware.com/forum/1/subindex/153

The original campaign, Shadows of Undredtide, and Hordes of the Underdark work just fine in coop. I don't know about the other included modules because I haven't played them.

I suggest picking one player in the party to do all the talking; occasionally the NPC's assume they've already talked to you when they actually talked to someone else in the party. It doesn't break anything except consistency though. If you have a bard or other player with high charisma and social skills, let them do the talking.

Each player can have their own henchman (2 each in Hordes of the Underdark). I have seen a glitch, though, where the henchmen in the party forget who their master is and randomly decide to follow a different player in the party. It's no big deal if you're just using the henchmen as extra firepower, but if the inconsistency bothers you, it's better to either not take henchmen along at all or to find a different module that handles it better.

Actually, you have to do a weird workaround to get everything to work correctly in HotU. Otherwise there's screwed up experience and stuff.

Okay I found an old post on the original game forums that helps a lot.

http://nwn.bioware.com/forums/viewtopic.html?topic=285858&forum=82&sp=255

Go down to the post made by bloodsong.

Basically multiplayer for all the campaigns is buggy and inconvenient with things like dialog problems and one player getting left behind when the other starts an event and moves on to a new area. The post I linked details how to start an MP HotU campaign where all players have the necessary starting items, and more importantly how to teleport players around for when you get forcibly split up.