I really love the concept of this game, though it still needs some patchwork, I think.
Most importantly, I'd say maybe speed it up a bit. Like some have commented on the page of the game, jumping & hitting would feel better if they were instant, as soon as you press the button. They idea of switching between 2 different characters while having to go through vairous obstacles as the screen moves, is very interesting and would make for an awesome fast-paced game! The interactions with the objects of the world are also a bit clunky at times. For example, in the picture I've posted, I can't break the rocks with fat even though they're just next to him. For some reason they don't break so I have to press ESC to restart the level, which can get tiring after some times.
I'd also prefer it if the controls were assigned to more common buttons. For example, jump on w & UpArrow, switching between characters on spacebar and q, e, or f for attack. I found that even after playing for a while, I pressed w instead of e or q just because I wasn't sure which one does what.
That being said, the fact that you did all of this on your own and for your first gamejam, is amazing. The atmosphere/graphics of the game is very nice and the ideas for all the different levels are interesting.
It's a solid game, just needs some more tinkering to make it perfect. Good job!