In the multiplayer plugin tutorials, linked in the manual article :
There is the notion of Host and Peer.
Your "main screen" is the host.
Any people connecting with their own phone is a Peer.
Hosts and Peers may display the same layout.
Nevertheless, thanks to Scroll To or the Scroll system expression/action you may position your camera on the Host to only display the "game area", and on your Peers, you disable the Scroll To of the Character and display a "Control" layer instead.
Then using messages (part of the Multiplayer plugin) you can have your Peer sending a message to the Host, the Host interprets the message and makes the Character move accordingly.