Dual lens dashboard video cameras (dashcams) are essentially two dashcams in one. Dual lens dashcams can record video from two or more lenses (or "channels") at the same time. This type of dashcam is typically placed in your front windshield to not only record footage from in front of your vehicle while you drive, but to also record footage of either the inside of the vehicle, or behind the vehicle as well. This means that you get video both of what other cars on the road are doing, as well as either what the occupants of your vehicle are doing, or what is going on behind your vehicle.
Some two-channel dashcams such as the AVIC HD-SL-Dual have both lenses built in to one unit. This style of dual-lens dashcam the most basic form of two-channel dashcam, and is usually intended to mount in your front windshield. One lens is intended to look forward (in front of your vehicle), and the second lens is located on the opposite side of the dashcam and is typically intended to record the activity that is going on inside the vehicle (the driver and/or passengers). Often with dual lens dashcams where both lenses are built in to the same unit, if careful consideration is given to where the dashcam is mounted, the inner lens that is built in to the main camera body can also "see" out of your back windshield, to capture a glimpse of events outside the rear of the vehicle as well.
Other styles of dual lens dashcams do not have both lenses built in to one module. If the second camera lens is not built in to the main camera body (such as with the BlackVue DR650S-2CH), this remote secondary lens may be placed just about anywhere throughout the vehicle, in most cases being mounted in the rear windshield and pointing out towards the rear (to capture events that occur behind the vehicle). This type of dashcam setup would give near 360 degree video coverage around your vehicle, for a truly comprehensive backup eyewitness. Or you could mount the second lens in the rear windshield and point it towards the front to capture a different angle of the car's occupants.
Who might want to use a dual lens dashcam? Fleet vehicle owners, concerned parents of a new teenage driver, taxi and rideshare (Uber, Lyft, Ride, etc), tow truck, or shuttle bus drivers, these are just a few examples where there may be a need to keep an eye on the driver or passengers of the vehicle, as well as the other cars on the road. Many a rideshare driver has used dashcam evidence from inside their vehicle to rightfully win damage and/or cleaning fees from an unruly passenger, for example.