I'm looking for a pre-made campaign fit to play with a single DM and a single player.
Expeditious Retreat Press has a whole line of 1 on 1 adventures
I like to give an NPC with equal Player Class levels to them (and one that compliments their class well, or one that can fulfill multiple roles) and set them up against any Prefab of their level-2.
I figure if the challenge rating system works for monsters it can work backwards too figuring that doubling the monsters adds +2-CR so halving the players would be mathematically similar. To start off I did some Modules for lvl 1 parties with a party of 2 lvl-3 characters (One NPC Paladin-3 and one Player Rouge-3.)
It worked well.
Expeditious Retreat Press has a whole line of 1 on 1 adventures
Count me as another recommendation for ERP 1 on 1's they are very well done.