Enter the amount of your outstanding mortgage principal on January 1, 2019, if you're doing your 2019 income tax return(s).
Per IRS Form 1098-Mortgage, instructions for Box 2:
I received 2 1098s as I refinanced in 2019.
It shows 0 in box 2/Outstanding Mortgage Principal of a of 1/1/2019 of the one they paid off.
What do I put in TurboTax?
Enter the amount you financed (the total amount of the loan) on the day you started the new refinanced loan.
According to the IRS:
"Box 2. Outstanding Mortgage Principal Enter the amount of outstanding principal on the mortgage as of January 1, 2019. If you originated the mortgage in 2019, enter the mortgage principal as of the date of origination. If you acquired the mortgage in 2019, enter the outstanding mortgage principal as of the date of acquisition."