Dual '2CH' dashcams are dashcams with two cameras. Dual 2CH dashcams come with a separate rear camera for the back of the car or with a second camera in the main body to record the interior of the car. The separate rear dashcam is usually (semi) waterproof and can be placed either on the rear window or on the bumper of the car. It is connected to the front dashcam with a long (usually 5-7m) cable. Only the front dashcam has to be powered, the rear camera gets its power trough the cable.
Dual 2CH dashcams record videos of both cameras simultaneously and save the videos separately on the SD card in the front dashcam. Some rear cameras can additionally be connected to the power of the reverse light. In that case, the image of the rear dashcam is shown in large on the front LCD screen when the car is put in reverse. This makes some Dual 2CH dashcams ideal to use as a parking assistance.