We have been discussing alternative reasonable methods to the guidelines in Pub 936. Not only must these alternative methods be reasonable, they must also meet the intent of the tax laws for deducting mortgage interest. In my opinion, you want to go beyond the reasonable and ignore the intent of the tax laws in order to claim a higher deduction. In any case, be prepared to defend the method you choose.
Please keep these points in mind: 1) You cannot apply the $750K balance limit individually to each mortgage. This could cause the $750K balance limit to be exceeded. 2) You cannot claim a deduction of the interest for both mortgages at any time during the year. This is the same as saying you had two main homes during that time. 3) Please check the calculations in your examples. There are several mistakes.
