Sharing a Screen
There are two ways to select the display being shared during a Tuple call:
Start a call with screen sharing
If you have multiple displays and choose to start a call with screen sharing, Tuple will prompt you to pick one before the call starts.
Just click anywhere on the display you want to share.
Adding or changing screens during a call
When in a Voice Call or if your pairing partner is sharing their screen, open the Tuple app by clicking the menu bar icon on the screen you want to share, then clicking "Share Screen".
If you're already sharing a display but want to switch to a different one, the text will be "Switch Screens".
If you're not seeing a menu bar on your external display at all, check that you have the "Displays have separate Spaces" setting enabled in Mission Control.