EMR have this rule on their 222 fleet, but not any other traction type.
A suitably safety trained first class customer host can be the person in the other unit. Occasionally an off duty manager, director etc or someone travelling to a meeting etc will step up, obviously in the correct way.
It's magically all ok through with multiple 170s coupled!
Most of these "rules" and "agreements" are relics of when drivers would need to leave the train to use a signal post telephone or around a guard not being able to reset a passcom alarm in the other unit without walking along track, potentially with 125mph running lines. Some, more pragmatic TOCs, understand that the driver can do this instead, from their unit, whereas at other TOCs, that would be seen as reducing the role of a guard.
Somewhat contrasts with Lumo running 125mph InterCity express services, completely DOO. Although they have a "ambassador" on board with a limited appreciation of the on-board systems and track safety, the safety case which these trains run on is pure DOO.