So maybe a follow-up question should be "What sorts of challenges or adventure structure lend themselves particular well to a duet game?"
Something that you would send James Bond to do, not the Marines.
Or "I have a unique skill set..." scenario
I see solo adventurers working well when …
- You have to be quiet and discrete (break into castle and steal / replace XYZ), a group would give itself away.
- The group was separated in the last session, bring in each part to do their thing until they get back to The Rendezvous Point.
- Jailbreak scenarios
- Lawrence of Arabia moves from place to place and visits different groups to set up local plans
- Side quest that is interesting to one player but not the others. Ex: Ranger will search for the Wizard's spell component roots & herbs in the forest
- Political juggling inspired by Cardinal Richelieu (Three Musketeers) -or- the Versailles court of King Louis XIV
- Player for whatever reason wants a combat-lite experience, more exploration and social interaction
- Personal revenge for previous slight / wrong. Need not be fatal; could be attack against reputation