If a customer holds both a Travelcard covering Zones 1-3 and another Travelcard covering Zones 4-6, there is no requirement for the train to call at a 'common station'; this is not in dispute (although some unknowledgeable RPIs have been known to make a fool of themselves by claiming there is a gap, as happened to
@jon0844 :
https://www.railforums.co.uk/thread...-combined-z123-z456-ticket.47857/#post-719413 ).
But those are two zonal products from the same family and range of tickets in the same ticketing system - the London Travelcards. You are simply continuing your journey within different zones of the same zonal ticketing system.
An M Card and GM County / Train cards are entirely different products which use different zone systems and are not the same zonal ticketing system. When you leave the WYPTE M Card zones you are not continuing your journey into different zones of the same system, you are using an entirely different product from an entirely different system.
In that case I would suggest that NRCOT Condition 14.2 applies, and thus both tickets need to be valid at one station.
"If you are using a Season Ticket, daily Zonal Ticket, or another area based Ticket such as a concessionary pass, ranger or rover in conjunction with another Ticket and the last station at which one Ticket is valid and the first station that the other Ticket is valid are the same, then the train does not need to call at that station for your combination to be valid.".