(I used to play a lot but don't anymore, was last at 1-dan AGA.)
What happens is that the weaker players become a little better at don't making reading mistakes and at finding possible moves. Unfortunately they don't get any better at evaluating moves and the unknown unknowns are still unknown. The better player finds moves that none of the weaker ones would think about or be able to explain.
I can imagine that the knowledge about the game of two 11 kyus have a smaller intersection than the ones of 6 kyus. This means they become much better together.
However two 1 kyu have about the same knowledge, so they could be at most 1 dan together, probably still 1 kyu.
I don't see it as much about knowledge of the game, more about avoiding mistakes and less pressure that lets you look at a game more in-depth. That, and two people will probably coalesce towards something like honte anyway.
Add to this the player opposing is under pressure because he HEARS the discussion. Can he take advantage of knowing plans? Should he?
I'd say this is a nice learning tool, and honestly, anything that keeps you interested and makes learning and playing a bit more fun is fantastic. I've played 5-in-a-row go, pair go, 3-player go, 1-colour go, 3D go and tried different board sizes like 38x38 or 19x19 infinite (sides 'joined'). I've never run across this variant! Seems fun!
Making guesses about what a 2-player team's strength is is a losing proposition, though.
Since we knew the opposing player was listening we mostly didn't discuss plans, at least not in a way that would have hurt the plans. Mostly we suggested moves to each other, and pointed out problems with the other person's suggestions.
I think 1kyu/1dan level is taken to mean that you've ironed out any major dejects in your game – as in, from here on out you're adding game lines:
A 1dan playing against two 5kyus will win if they combined still have overlapping defects, depends on the overlap. It'll be closer than one 1dan versus one 5kyu but I'd still expect the 1dan to win.
I would have expected a 6kyu to defeat combined 11kyus, so this is surprising. Would a 6dan defeat combined 1dans?
We should set up two-headed games with varying strength differences and find out, would be super interesting to see what the win outcome rates are.
Plus, the 11kyus might be underestimating their rank. You can get better pretty quick, and might not realize it.
We've both been around our current level for years. My dad plays somewhat regularly with a small go club, while I mostly play my dad a few times a year.
I do think it's likely that if I either studied some theory or played a bunch of games against people slightly better than me that I would get better quickly. I'm a relatively non-central example of someone with my approximate level: I've been playing for about 20y, but almost all of my games have been against my dad.
Remember that it's not allowed any talking, except asking who's to move, maybe who's to take a ko, and proposing to resign to the mate.
(To be clear I'm only talking about the two players on a team part, it's usually 2:2, not 2:1)
Personally I hate it and am absolutely awful at it, but to each their own.