Played and finished! ) Se you really like your negative-space games ;)
A solid concept! Seen similar ones before, and most the the levels also shared a lot of classic elements with moving "around" the screen to connect to the right side.
I can see that the idea most likely was that BOTH characters where suppose to enter each of there doors, so I will judge it on that (otherwise the mirror world is rather pointless), taking that in account the levels are relatively challenging as you go along.
What I did miss was just something more to flare out the otherwise rather simple levels. Its hard to create that in 3-hours, and honestly just the amount of levels is impressive for that time (+ the transition, love it!), but just something that adds to that mind-bending-feeling I want from these types of games. I think the grey blocks was a great start, but only the loop-around-level really used it to any effect. I would have loved a feature/mechanic that's similar to the glitch transition effect.
A fun play :) Make more and take it to the next level!