Pretty sure you should be able to play even 2 Dark Urges, you're technically 2 different players so yeah, you can play Tav and DU on multi for sure.
Do you have a source on that? In the character creator Durge is treated like an origin, so if we can have two of them it would kind of be like having two Astarions or two Shadowhearts. I genuinely have no idea of how multiplayer handles this and haven't seen it discussed anywhere, though it's an excellent question.
EDIT: I see benbaxter was in the middle of saying much the same thing as I was writing this
