very, very fun brain-wiggeling experience for single player. it would probably too easy for coop, but it's still cool, that you put the option in. I got some flashbacks to playing "Brothers - A Tale of Two Sons" which is one of my all-time favourite games.
The music is called "Space Jazz" by Kevin MacLeod. I know this, because I used the same track in my last years gamedev.tv gamejam entry - which felt really bizarre when I heard it in your game again ;-)
if you'd like to have a similar experience, here it is:
https://kindman.itch.io/life-after-death