Co-owners on a house exceeding total mortgage interest deduction limit but under limit if splitting loan amount

I'm currently full owner of a house with a $300k mortgage with interest payment of $10k. I'm also a co-owner of a house with a $800k mortgage with a total interest payment of $18k with my share at $9k (50% responsibility). Turbotax has me putting $800k and $300k as the mortgage loan amount (since instructions indicate to put what the form says) and interest at $10k and $9k (my share) respectively. However, due to the $750k limit on loan amount you can deduct interest from, it claims less than $19k in interest deductions since it thinks my total loan amount of $1.1M exceeds $750k. Since the 2nd house is co-owned, shouldn't I be under the limit (300k + 400k instead of 300k + 800k) and claim the full $19k in interest payment deductions? Do I need to put half of the loan amount instead in Turbotax?