Hello,
This section acts as a brief tutorial for switching between the two characters. To proceed, the two characters must both stand on the center of their respective platforms. As for why they are off the platform, it could be a minor bug when spawning into the room, as there are collisions set to prevent moving off the platform. If you are unable to reach the platform, you may need to restart the game from the previous checkpoint.
Thank you for checking out the game. Good luck! :)