The Dual Shot feature was first used in Galaxy Device where both the rear and front cameras would took a picture at the same time. To take dual shot on LG G4, do as follows:
1. Open the default Camera app on your LG G4
2. Touch the Mode option from the top menu
3. And then tap to enable the Dual mode
4. Now as you can see, both the Rear and Front views are seen.
5. Tap the small arrow on the left middle to open the customization panel and select your desired layout
6. Tap the small screen to switch the cameras
7. Tap and hold the small screen to resize it