When you connect two iPhones together to record using Detail iOS and the Multiplayer feature, the app will automatically change the layout to Picture in Picture, and on both devices you will see a camera 1 (device one) and camera 2 (device 2), each of which will either be the showing the front or rear camera of that device.

To swap from front to rear camera on either device, tap this camera toggle icon (🔄) and the feed will change from front to rear or vice versa.