My dad and I are a similar level (~11kyu) and last week a family friend who's substantially better (~6kyu) was looking for a game. Instead of either of us playing him 1:1 with ~5 handicap stones, we decided to play 2:1 even. As a casual game, we played untimed.

It was a really interesting game: my dad any I both compensated for each others weaknesses, were more thoughtful and careful, didn't make any major mistakes, and ended up winning by a lot. It felt a lot like pair programming.

I'm very curious now how much better playing as a pair tends to make people. While I expected us to do better, +5 ranks (+500 ELO) was a much larger change than I would have guessed. It also may not be linear: I would expect better players to generally benefit less, since worse players are more likely to have uneven-but-complementary skills?