First make sure you are connected. To connect to another device tap the button of a person and a + icon, you can find this in the bottom right of the Home screen or the top right of the Project screen.

To connect 2 devices remotely or in the same room, tap this button to share an invite link with someone. Connect when your guest opens the link & joins.

When connected, tap the record button on either device and both will start recording. Once the recording stops, high quality footage will be synced, letting you create a high quality export.