I guess it's possible to take NPCs along in a multiplay session (there will be two of us playing by LAN); but who has control over those? Both players or just one?

The Host of the game can define that.

So basically you can define any of your human comrades to have control over any of the npc:s, at any point of the game.

Basically it means that if you got "You" and "Him", and Minsc + Jaheira + Imoen, you could just define it so that "Him" has control over Minsc and Jaheira, whereas you have control over Imoen. This way neither of you will be able to control each other's characters. But if at any point you want to switch who controls who, you can do that by just going to the multiplayer options screen and change it from there. Even in the middle of a battle. But remember to pause first of course. :)

Thanks, I'll check that before playing... also, does hosting mean you have to play the protagonist or not?

You have to play the protagonist i think.