You deduct property taxes when you actually pay them, not for the year they represent. With the additional requirement that the taxes must be legally assessed.
In your cases, if you paid the 2019 tax bill in January 2020, that's a 2020 deduction. If the 2020 tax bill is legally assessed in June 2020, you can pay it any time after that and, if paid before 12/31/2020, would also count as a 2020 deduction.
(Where this becomes an issue is, for example, New York State, where county taxes are legally assessed on January 1 and due Feb 15. Paying the February 2021 amount in December 2020 would not be a legal tax deduction since the tax had not been legally assessed at that point.)
