This is a bad solution because you end up getting a lower deduction than you're entitled to take, if both mortgages are above the 750,000 limit. That is, if you pretend the two mortgages are merged into one 1098, TurboTax won't account for the lower balance on the second mortgage, and as a result, when it divides the 750k limit by the higher balance, the result is a lower percentage applied to your total mortgage interest paid in 2021. In other words, you don't get to deduct as much mortgage interest because TurboTax (incorrectly) thinks your mortgage's balance is further from the 750,000 limit than it actually is.
