You press up to enter the locker with the bottom character. To exit the locker, the top character needs to be in the same place as the bottom character and you press down to exit the locker and continue.
this might be a bug but i did not get hit by the enemy i did not get to enter the locker and when pressing down or up and left or right arrow keys the game seems to be strange all movement stops and you can walk with one character only. That is how i managed to get through the level, the bottom character never passed the enemy ;s