nice game. I liked the story, and the overall idea of one of those sliding puzzles but one of the characters can be controlled to go anywhere is pretty cool in concept. the level designer is a nice addition as well. its not something usually seen in puzzle games.
For the issue with the difficulty, I think there are a few ways I can think of to be able to make this harder without just getting rid of the teamwork aspect.
I think having permanent that only stop one but not the other would be one way of solving this. Since this wouldn't prevent them interacting in general, it could allow for much more possibilities. I think in the level design, having tighter paths could be a method of making it more difficult, since cappy can't just be positioned anywhere.
having a counter for the amount of moves could be a nice addition as well.
I think there's probably some way fit the name of what character you're controlling, because the current system isn't as clear.
the leaver system is also kind of confusing. still didn't figure out how it works after beating the game.
Finally, is that glitchy effect when transitioning between levels intentional? If it is, then I think it should be something that can be turned off in the options because of the flashing lights.
Update, i've used the level editor and there are a few problems i've noticed. sometimes replacing a solid wall with nothing doesn't work. also, in the text editor you can't see anything past the first line in each text box. also, I think there should be an option to make it mandatory to collect all the trash to win.