R did not think ahead about his day's public transport travel. If he had, I would have worked it out in advance. We boarded a tram and he validated his ten trip ticket. It was the last trip on his card. When we arrived at the city to change trams, he decided he would duck across the road and get another ten trip ticket. I saw a problem.
After your second validation of your two hour ticket, you are not debited again. You have already paid just a little more than the value of a daily ticket. But if your second validation is via another ticket, then the system will not know it is you and will debit another two hour trip. I knew he would have three seperate two hour trips that day, so after one validation of his new ticket, we agreed that he would just keep the two tickets as proof of payment of two two hour trips, should he be challenged.
Funnily, later I was challenged by an authorative person for my ticket, but R just walked straight past them.