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.
If you look at the character selection menu, there is a separate section for Tavs and from Origins. Durge in is the Origin section, so it is unlikely that you can run two of them, just like you can't run two Shadowhearts or Gales. Even though Durge doesn't have a specific race or class, they are a very specific and singular character.
