Does the MacBook Pro need both Ambient Light Sensors to function correctly?
On my 2015 Macbook Pro, what looks like two ambient light sensors is actually a light sensor on the left of the webcam, and a green LED on the right of the webcam that lights up when Facetime is open to let you know the camera is in use.

The sensor on the left can be tested by waving a flashlight in front of it and observing the keyboard dimming. The light on the right can be verified by opening Facetime.