To add another camera to your green room, use the “Add another camera” button beneath your stage. Once the selection modal appears, enter a name for the camera, select the camera you’d like to add from the drop down, mirror it if needed, and then save your changes. Once you've added it to your Green Room, you can then add it to the stage by selecting it from the list of cameras on the left. You can also adjust the new camera’s settings from the camera preview on the left or remove it from the Green Room altogether.
Note: Currently this option is camera only, so no microphone will be added with it.