You treat the two loans separately.
You can deduct 100% of the interest on the old loan assuming it was acquisition debt. If the loan included equity debt, you have to make an adjustment.
Your new loan was $784k for the entire time you had it. The only interest you paid was daily interest on your closing statement from 12/18 to 12/31. You can deduct 750/784 or 95.66% of that interest.
