The Duo is not technically an Archetype at all, but ships of other Archetypes designed for two-player local Co-op. All Duos are currently Hybrids of Duo and one other Archetype.
No unique nodes; Duos borrow nodes from the other Archetypes, including Veterancy.
A Duo's distinguishing characteristic is that abilities can also be bound onto a second controller (though you can keep them all for yourself if you want) and that the ship itself has two rotating parts. One is under the control of the pilot, player one, and the other is controlled by the gunner, player two.