I think it happened that I was so into the "zone" knowing what happens I forgot to better explain the mechanics...thats what happens if you are the only one playtesting!
So the mechanic basically is that if you touch a wall with a player, it gets solid for THAT player (leah or main character), and the other one can pass through. In the Leah 1 Level, you can jump up with the Player, then Leah can fall through the base box because it is solid for player, but not for Leah. As you now need to get both Leah and player to the exit, you need to think how to well do that :)