Today I ran my first session in a duet campaign for 5e D&D. My player wanted to play an investigation-focused campaign, so I built an investigation based on one of the one-shots in the Defiance in Phlan Adventurers League module, where instead of being told the results of the investigation and being sent to the dungeon, the player gets to also do the investigation part.
The majority of the session was walking around town talking to people, in a way that would probably be less engaging for a full party, but is very fun for a single-player campaign, since the investigation is fully player-led.
A duet definitely takes a lot more out of me as a DM, since there's less break for me where the players just talk amongst themselves, but also because there's a lot less intraparty debate, things move along a lot faster than in a game with a full party.
No combat today, so no reports on balancing combat yet. My player is my regular table's Forever DM so I can't tell if what I ran was actually fun or if he was just so delighted to be a player for once that he would have had fun if his PC was placed on a railroad and watched paint dry for two hours.