I also think it is intended.
I remember Swen talking about the possibility in MP to backstab a NPC while one of your mate talk to him.
Ohhh! NOW I remember in my first multiplayer playthrough, my husband snuck up - controlling Astarion - behind a goblin NPC I was in the middle of talking to, and stabbed the goblin, one-shot killing it "because it's what Astarion would do". It was hilarious at the time.
However I do think there should be time limits imposed and you shoudln't be able to just leave a character stuck talking to an NPC forever, the world should still continue and react to other characters...
