Hi Laura
They can't break your contract on the investment property mortgage.
If your home is in positive equity, they should be delighted to get rid of the cheap tracker on the investment property.
If you home is in negative equity, they may make it a condition of the sale, that you pay some of the proceeds of sale of the investment property against your home loan.
Investment properties with cheap trackers are usually extremely profitable. You should be very slow to sell it.
You probably should not be overpaying the tracker on your home.
Had you not been overpaying for the last 10 years, you would now have a lump sum, say €20,000. This would be a huge incentive to ptsb to allow you to sell your investment property in return for paying this off your mortgage.