You would have to be paying more than €10,000 per year interest to realise the TRS benefits of having two people on the mortgage over what a single person would get. Right now, you are each paying €4250-odd interest per year and are each getting TRS on that amount. If you were paying €20,000 interest, you would each get TRS on €10,000, whereas a single person paying the same amount of interest would only get TRS on €10,000