Take each house's property tax bill for the year, divide that value by 365 days in a year to get the daily property tax...
Then calculate the property taxes you paid on each house by taking the number of days you owned the house times that daily rate.
E.g., if you own house 1 for 100 days and its daily property tax equals $10, your property taxes on that house equal $1,000.
If you own house 2 for 200 days and its daily property tax equals $20, your property taxes on that house equal $4,000.
Your property tax deduction on Schedule A combines these two amounts... so $5,000.
BTW, you can read through the closing statements and mortgage interest statements, pull numbers off of them, and come up with a very, very similar value.