If you have more specific questions with examples that you've been testing with, then for sure it's going to be easier for people to help you.
While it might sound stupid to begin with -> just rotate EVERYTHING.
You can turn an entire layout using "Set layout angle to 180°"
You can turn sprites using "Sprite - Rotate 180° clockwise"
This way you would have to rotate everything, including individual HUD elements. The good thing is that, it allows you to sync object positions through the multiplayer plugin. The HUD will need some additional positioning adjustments.
Another way to do it would be by only placing the objects to their respective top or bottom position based on if a player is the Host or a connected Peer. For that, look at the Multiplayer tutorial, there you can find everything on how to differentiate between the host and the peer. This would mean that you can't sync object positions using the multiplayer plugin though. Instead, you just mirror the object positions for the Peer. The HUD can also be easily manipulated by differentiating between Host and Peer.