Duel-Camera options is to choose the behaviour like Camera app and let the system decide automatically choose which camera to use during capture and can combine data from both cameras to improve output. Also it does not support any RAW photo capture, OIS, HFR and Slo mo.
For more info check iOSDeviceCompatibility
From Documentation:
Dual Camera
Some iOS devices contain a dual camera—two separate back-facing
cameras that can work together as a single capture device. When
setting up a capture session on a device with an dual camera, you can
choose to use only the wide-angle camera, only the telephoto camera,
or the dual camera. Choosing the dual camera capture device provides
behavior similar to the built-in Camera app—the system automatically
chooses which camera to use during capture, and can combine data from
both cameras to improve capture output.
The AVCaptureDevice devices and devicesWithMediaType: methods are
deprecated in iOS 10 and do not provide access to the dual camera
device (instead, they provide only the wide-angle camera device). To
determine whether a device contains a dual camera and select it for
capture, you may either:
- Call the AVCaptureDevice
defaultDeviceWithDeviceType:mediaType:position: method, passing the
builtInDuoCamera device type. If this method returns non-nil, the iOS
device contains a dual camera. (If this method returns nil, you can
call it again, passing the builtInWideAngleCamera device type, to
obtain the default back camera.)
- Create an
AVCaptureDeviceDiscoverySession object, passing the device attributes
you want to use for capture, and enumerate its devices list to choose
a device for your capture session.
When you use the dual camera
capture device, Optical Image Stabilization (OIS), RAW Photo Capture,
High Frame Rate (HFR) / Slow Motion Video, and most manual controls
are not available. To use these features, specifically select either
the wide or telephoto capture device. (For OIS, specifically select
the wide-angle camera; the telephoto camera does not support optical
image stabilization.)