Two tickets, but one plane is the saving motto I think. The idea is that Ticket 1 from LHR to AMS flies on plane 1. Ticket 2 from AMS back to LHR (and then onwards elsewhere) is also scheduled on plane 1.

If Ticket 1 is cancelled because plane 1 is cancelled, then plane 1 does not fly to AMS. THEREFORE, Ticket 2 must also be rescheduled because there is no physical plane at AMS to fly to LHR. Plane 1 never made it to AMS so cannot depart from AMS, so your entire Ticket 2 itinerary is disrupted.

Hence, the perverse rule that a b2b saves your ENTIRE itinerary.