The ticket system is set up that basically you can only do one of two things to an existing ticket.
One is to reissue a ticket, usually done when the magnetic coding has been wiped out or to switch between a paper or plastic ticket.
The other is to upgrade a MYW ticket to add more days or options.
In either event, the system will only allow one ticket as the input and one ticket as the output in one transaction.
The only "exception" is if you are upgrading multiple tickets at the same time, such as adding Water Parks Fun and More to base or hopper tickets, where there might be a family upgrading four tickets and the result will be four new tickets out but only one payment to be made.