If you are not married by Dec 31 you each file as Single. Don't worry about deducting the mortgage interest in 2020. You probably won't have enough interest to itemize. You will probably each take the Standard Deduction. Unless someone has enough other deductions like medical to itemize.
For 2020 the standard deduction amounts are:
Single 12,400 + 1,650 for 65 and over or blind (14,050)
HOH 18,650 + 1,650 for 65 and over or blind
Joint 24,800 + 1,300 for each 65 and over or blind
Married filing Separate 12,400 + 1,300 for 65 and over or blind
