Unfortunately, it is a problem to try and run the same MileIQ account on two devices at once. Having the same account on two devices can cause quite a bit of confusion as the departures and arrivals can be in multiple places simultaneously, creating all sorts of errors.
We are looking at adding group accounts that will account for use-cases like this, but in the meantime, please delete the MileIQ app off one of the devices (or sign out and sign in with a different email on the second device).