I ran a two player campaign back in 1E, where they had 2 NPCs with them for several adventures, but not all the time. In general I'd focus more on the social and exploration tiers than the combat tier. Investigation adventures are far more interesting, and don't need nearly as much combat as other adventures. Enemies in combat are best if at least semi-intelligent, because they may take hostages if both PCs are dropped to 0 HP (which can happen a bit more easily with fewer players), preventing a TPK that ends the campaign. I'd choose to use milestone leveling or provide xp for non-combat challenges to ensure it doesn't take forever to level, due to the limited combat.