This is a really tricky game! It takes a lot of my brain power to try to keep both slimes' movements in my head, which makes for interesting gameplay.
I will say that kicking the player out to the main menu seems a bit harsh; it sort of messes with the cycle of trying different techniques and dying hundreds of times doing it (ala Super Meat Boy). Perhaps this is not what you intended though!
(Though if the game is played with the keyboard, make the menu navigable by the same! Otherwise the player has to reach for the mouse every time to restart, and I think reducing the effort it takes to play again to as little as possible is crucial if a game is this hard!)
Anyway, nicely done!