Pressing M will spawn in the second sprite using the old engine. Press space to swap environments. You can drag both sprites around.
I just recreated my old physics engine and changed the way it calculates movement and other stuff. I also figured out a way to make it work with anti-gravity though, it isn't very consistent. If anything looks wrong, tell me in the comments. Old project: https://scratch.mit.edu/projects/468426344/
