if dual camera is there..one is main camera another is telephoto lens...you can't capture from both camera together at a time..you have to choose any one of them
open camera , than swipe right to left or goto options pro , sports mode etc there you will see dual camera mode , mention your device modDual camera mode captures with both cameras at same time , But while previewing the video you need to careful about the option , cause at preview it will show you one video at a time if You dont hold the tiny dual view icon over video and than press dual view option as attached below
or check this option on your device after stoping the dual camera mode recording dual view option as attached below
