Amazing game, especially for a game jam. The art and sound are simple and small scope but well executed and I think that was exactly the way to go. The main mechanic makes a lot of sense, and while it seems obvious, you guys combine it with many other elements creatively and that counts for a lot! Surprisingly I haven't see a lot of games use this as the main mechanic, probably because they couldn't make it fun, but you guys did. The onboarding process is pretty minimal but it's enough to get the player started which is good.
There were a few instances where I didn't understand how a puzzle worked and had to experiment, especially when it came to more platforming dependent puzzles, and I ended up finish it just by going through my options. This means I hadn't learnt the mechanic to beat that puzzle beforehand well enough. There were only a few cases like this so it's not a big deal, just something to note.
The game starts out pretty great and for a few levels it feels very clever in level design and mechanics. Then, as it goes on longer, it starts to rely more and more on timing-based challenges largely due to the heavy gravity applied to the character. These are interesting level designs too, but they are challenging in execution rather than in puzzle thinking, so the game starts to feel less clever and more tedious. I don't think this is a huge deal because I feel like you guys do enough already in the first few levels to make a fun game jam game. But just a suggestion in case you guys continue to work on this game, which I think you definitely should :).
Another thing - the game is very condensed in terms of puzzles. It's puzzle after puzzle after puzzle after puzzle. Even within a single level it's puzzle after puzzle. I liked the puzzles a lot, but this causes some puzzle fatigue. It makes me feel like I am constantly moving at slow pace, and you even see this from the very beginning where it takes a while to get past the first level. I completely understand that this wasn't possible for a game jam, but if you continue working on it - I would space out the puzzles with easier sections that do fewer puzzles and more easy platforming, or just easy puzzles. I could also seem some simple enemies being implemented which get damaged due to the join / split mechanics and this could also break up the pace a bit. You guys definitely went for the puzzle platformer route which is cool. I suggest looking at Prince of Persia Sands of Time for good pacing for a puzzle platformer. That game maybe has too much combat, but in general they do a good job at balancing out puzzle platforming with non-puzzle platforming with easy combat sections.
Overall, I think this is actually the best game I've seen in this game jam, seriously. Please keep working on this after the jam, it's awesome! Put it on steam!
I would love your rating and comments on my game as well, so I can know what to improve!