PUB 936 speaks to average balance but without clearly defining how to compute the average when a loan is taken out during the year. I think an acceptable method would be for the first home to average the balance at the beginning and end of the year by totaling those amounts and dividing by 2.
for the second house, take the beginning balance and the year-end balance and divide by 2
multiply this by the number of months you paid interest in 2024 and divide by 12
divide 750 by the sum of the two average balances and multiply the total interest paid
so you might have something like this'
average balance 1st house 750K average balance second house $100K because it was taken out late in the year
deductible interest would be 750 /(750+100)* 27
