There's nothing stopping GNE and Arriva/Stagecoach having mutual ticket acceptance on the 684 and 685, it's just a case of them thinking that a timetable change is "joined up thinking" and leaving it at that, in reality it'll only benefit ENCTS holders and those happy to pay for single tickets on a round trip.

For those who hold a single operator's day or period ticket, a half hourly combined service is pointless.