Lovely design, it felt really effortlessly beautiful while also being extremely readable for the mechanics! Only design work I'd suggest to tweak would be a more nuanced way of presenting the numbered buttons so they don't break with the aesthetic quite so much.
While the first two levels are expertly balanced, the third level is waaaaay too hard. Like some of the comments below I think this is simply because of how hard it is to trigger the double jump, as that saw me failing many times (because I couldn't double jump) even though I was jumping in the right place at the right time. I'd generally say with platformers like this you'd want to make the hitbox larger than the object to add that little bit of leeway for the player.
It's also worth noting that I never moved both characters at the same time, as that seemed like the easiest way to play, but you could really crank up the complexity of the game by making that necessary for some of the puzzles.
But yeah; if this were a full game on steam or mobile I'm sure it'd do great!!
I'd love to hear your thoughts on my game: https://itch.io/jam/brackeys-5/rate/928249 :)