Must agree with OP. It's not too effective for both devices to count when they're together. As both have motion control components and chips, it would be more reasonable for each device to know when it's on the go and prioritize that device. All we are told is how clever these newest chips are and how they can make billions of complications without sweating, surely this is another they can do without too much effort.
Similarly, when an active mobile phone is away from a phone, but the phone is still active with a signal, the time is NOT registered / available and can not be selected to be. This again is a bit odd.