I'd argue that there's many games where this would be perfectly valid, since there's plenty of genres where a pair of adventurers works well. Homes and Watson, for private detectives; Starsky and Hutch (dating myself) for buddy-cops; Fafhrd and the Grey Mouser, for sword & sorcery; Batman and Robin, possibly, for supers. Not all systems support it as well as others, but there's a surprising number that don't require "varied characters" in the way some where the rules are quite rigid in limiting what characters can do. Some that I've played with that number of players and which worked well:
Pendragon. A pair of knights errant, wandering around the countryside attending local tournaments and occasionally going on quests to help locals is a perfectly normal way to play the game, genre-appropriate too, and it's not hard to justify someone joining them temporarily (a meeting at a tourney, on the road, or while receiving hospitality, and there you are with an old acquaintance).
Barbarians of Lemuria, Conan 2D20, or something else designed for Sword and Sorcery adventuring. Conan is famous for travelling alone but Fafhrd and the Grey Mouser are almost always a pair, Thongor travels with several companions, and temporary allies are a frequent feature. Some of the BoL spin-offs such as Honor and Intrigue also work, depending on the genre.
Heroquest, but most supers games should handle it. I was using it for supers, two players made a crime fighting duo, occasional guest appearances from other heroes. A gadgeteers and a martial artist, they fight crime together. Just like Angel Summoner and BMX Bandit, but equally useful.
