A pretty cool concept! A few times while I was playing through the levels I really wanted to switch which character I was controlling without merging, which I think would've opened the door to a lot of potential uses and puzzles. Also not a bad decision to have it so you can't switch without merging, cause it certainly made me think in a different way!
I was able to essentially skip the last two levels with some carefully timed jumps and splits to have the ghost almost go straight to the end, but other than that, I really liked the puzzle design from this concept. Good work!