To cope with some of the earlier problems mentioned on this thread, if I was designing it, I would resolve them at the time of loading the ticket onto the card.
Presumable when you buy a ticket you have access to a device that can ask you questions. It would be "simple" to to analyse the combination of tickets and say "You already have a ticket that would be valid for some of the journeys your new ticket will allow. Please indicate which journeys you would like to charge to which ticket."
Presumable when you buy a ticket you have access to a device that can ask you questions. It would be "simple" to to analyse the combination of tickets and say "You already have a ticket that would be valid for some of the journeys your new ticket will allow. Please indicate which journeys you would like to charge to which ticket."