Hey, really cool game you’ve made here I really enjoyed the music and game is quite fun to play! I like the concept of having two different characters that you need to get to the end with.
Some pain points for me were, as you mentioned in your description, the double jumping and lack of checkpoints. Double jumping was quite tricky to figure out and I found it kind of hard as I would move away from the wall before I could jump off it. Also I think the game would benefit from some checkpoints as it is a wonderfully challenging game, but can be very frustrating when you make it far and have to start from the beginning.