This post is from the Activity iPhone Duo Photos & Camera Q&A.
Can all the cameras on iPhone Duo be active while an app tries to translate sign language using the front facing camera and back camera to monitor hand movements and show the translation with action in the expanded display with its camera on?
You can have an AVCaptureMultiCamSession that is configured to use a back camera and a front camera together.
You'll find in AVCaptureDevice.DiscoverySession that supportedMultiCamDeviceSets indicates support for three physical cameras running simultaneously.
Since the camera direction can change, use the AVCaptureDeviceDirectionCoordinator to know what the direction is for a particular camera at any given time on iPhone Duo.