I have a similar situation. Using the add another lender as described in TT, I entered the one 1098 mortgage interest statement as two entries - one for personal and one for the months corresponding to the business. I entered the outstanding principle under the personal entry and for the business entry I didn't select "My form has info in box 2, 3, 7."
When I finished my federal return data entry and when through the checking process, TT required that I fill in the outstanding principle balance on the entry for the business interest. When I applied the same outstanding principle balance, TT recalculated my refund into me owing taxes.
I therefore, went back to both mortgage interest entries and split the outstanding principle balance based on the same percentage as I used for the mortgage interest. This resulted in no change in the TT refund that I was expecting.
Of note, others seem to be having issues with this too. I also noticed that the rounding for mortgage entries doesn't round correctly.
