I like the idea a lot.
It's a very simple, but effective mechanic that's explored throughout some levels.
A bit more polishing would have been nice (e.g.: No friction between the two players, to make it easier to move stacked player characters horizontally)
It took me a while to figure out how to move efficiently and how to stack the player characters. Once I figured it out the remaining levels were pretty easy and it felt very good to nail the execution and see everything play out.
For a post jam version (If you plan to make one) It could be cool to have even more ghost players and some torches in the walls (or something) that make them visible too. This way the potential for puzzles would open up immensely.
This is a very solid entry with a very cool interpretation of the theme!