That's a lovely concept, and the execution is fairly decent. I liked the idea that you need to make use of the abilities to move around instead of walking and jumping. I liked that the NPCs were not too fast. I also liked the feedback on checkpoints- the trees growing. That was really immersive and super cool!
It took me a while to comprehend how to effectively use the stretching mechanics . Also, I think when the character(s) fell off a platform, they just floated there, allowing me to rotate back up to the ground. Idk if that was a bug or not, but it did break the immersion a bit.
A few suggestions that come to mind are-
1. Set distinct and contrasting colours for Jack and Jill. Often it was hard to tell which was which.
2. Ease up on the difficulty curve.
a. Let the players get used to the mechanics before introducing enemy NPC.
b. Instead of introducing both abilities at once, you could introduce one after the other, and then mix them both after a while. (but yeah it needs to be playtested)
3. It would be cool to have an option to manually switch between the characters (even when they are mid-air, so that the player would have better control. But yeah this needs to be playtested as well)
Overall, it's a super cool game, and with further polish, it can be even more enjoyable. Nice work, and good luck!