My resolution:

Put "ending balance" for loan 1(seller of loan) as zero (0) for 2018.

Put  balance for loan 2 (buyer of Mortgage ) as principal balance. (Value is required even though 1098 , box 2 is empty)

Loan Origination dates were the same for both lenders. (This indicates it is a grandfathered loan and not originated in 2018)