If you have experience with such a Duet, what are the best practices and pitfalls to avoid?
I don't know the Dragonbane RPG, but have done this sort of play a reasonable amount.
Most successfully, I've played quite a bit of two-person Burning Wheel, some with a single GM/single player arrangement, and some with the two of us playing and GMing simultaneously (in this latter case, each of us is in charge of framing the situations and establishing failure consequences for the other).
When I've GMed for my friend, he has played a single PC but with a developed cast of NPCs around. When he's GMed for me, I've played my PC with a companion (as per the BW wheels for creating an apprentice/sidekick). Generally I play the companion as well as my PC, but when the two characters argue the GM has played the companion (within the limits/context of my characterisation of that character).
When we play two GMs/two players, there are our two PCs, sometimes in a cooperative relationship and sometimes somewhat adversarial.
I think Burning Wheel is well-suited for this sort of play because of its character-driven approach to how situations and consequences are established. But I've also done more adventure-based RPGing in one player/one PC mode. I've run Classic Traveller for single players, each with a single PC. It does rely on the player being fairly proactive in picking up on cues and using their character to drive the action.
I think any GM-controlled NPCs definitely need to be secondary/background/responsive-rather-than-leading in this sort of play. Otherwise it really risks being the GM playing solo, with the player as audience and occasional contributor.