Wow, what an interesting game! I like pretty much everything about it! The mechanic of playing around with different combinations and finding out that some things are way better for certain situations than others was like I was discovering fire!
The levels you guys built over only three ghosts already shows the huge potential this mechanic has. The few things I'd like to point out is that it could be interesting to put order switching for the ghosts so we don't have to redo the line in the case of a change, and staying too close to the walls seems to make the collision get a bit freaky for jumps. Other than that, this is a really promising game and I can easily see a more expanded version of this existing.
The artstyle is really endearing and funny, the tiny details like the mix of the ghosts expressions when they're fused are a very nice touch, and the music fits really well with the vibe you guys were going for regarding the catacombs.
Awesome work!