i double manned for almost 2 years and let me tell you that there isn’t a person out there that can stay awake for the entire 22 hours that a double man team can be on duty for, if the driver not driving was unable to sleep whilst the other driver drove then there would be a hell of a lot of accidents.
what it says is that the driver resting may take that rest period in a bunk or couchette whilst not driving but the vehicle must be stationary when both drivers rest.
some coaches actually have a bedroom that can be accessed either from the outside of the coach (usually refered to as a coffin by regular drivers) or that can be accessed from the stairwell near the toilet.
both my partners when i double manned used to sleep when not driving as did i.
it is sometimes difficult but without that sleep the drivers would be a danger to other road users.