from the Shortcuts Toolbar and tap Select Display:
If your Mac is connected to more than one Display, it is possible to select which display you wish to use:
Tap on the Actions button from the Shortcuts Toolbar and tap Select Display:
Tap on the desired layout:
Screens will only show the selected remote display or all of them if you choose that layout:
TIP: You can have Screens auto-select the last used display on connect in your screen settings.