The constructing-the-slimes part definitely worked quite well. The overall design of the game made it feel like having two characters wasn't tacked on, and the credits scene was very cute :D
Some constructing parts did feel like they required a bit too much precision, which was difficult with the fast acceleration of the slimes, e.g. with the slime being below the add-on, and having to make it hang-on to the next jump.
Overall, I reckon this is a very elegantly designed game. Good work!