They use two so that if you rest the airpods on table in various positions, at least one IR sensor is pointing away from a surface. They turn on and off the dev

They use two so that if you rest the airpods on table in various positions, at least one IR sensor is pointing away from a surface. They turn on and off the device (pause, play) once both IR windows are covered by human skin (or detect a surface) which is the scenario in which they are fully in your ear. If they used only a single IR, the function wouldn't work if taken out of the ear and say laid face down on a book or table.