Assuming both mortgages originated after Dec 2017, the interest deduction is limited to $750K of the combined average balance. After entering the 1098 for the first mortgage, the tax liability reflects an interest deduction of $30K (the amount of interest paid on the primary home). After entering the 1098 for the second mortgage, the tax liability reflects an interest deduction of $750K / ($640K + $760K) * ($30K + $20K) = 53.6% of $50K = $26,800. This is the Simplified method of Pub 936 Table 1 that Turbo Tax uses. The $750K limit must be applied to the combined average of both mortgages when using this method. You don't get to apply the $750K limit to the first mortgage and up to another $750K limit on the second mortgage.
However, if the mortgage on your primary home ($640K average balance, $30K interest) has an earlier origination date than the mortgage on your second home ($760K average balance, $20K interest), you will benefit greatly by using the Exact method. With this method, the $750K limit still applies to the total of both mortgages but it is split and applied separately as follows: Since the average balance of the first mortgage is less than $750K, the limit is equal to it's average balance ($640K) and all of the interest on this mortgage ($30K) is deductible. The limit for the second mortgage is equal to the remaining limit ($750K - $640K) = $110K and you deduct an additional $110K / $760K = 14.5% of the interest paid = $20K * 14.5% = $2,900. The total interest using the exact method is $30K + $2.9K = $32.9K.
Remember, you must take the mortgages in the order of origination date. If your second mortgage has an earlier origination date, your deduction would be $20K.
