You can't compute the average balance for the primary home (home A) as $650K /12 *8 = $433K because it wasn't a qualified home for 12 months. It's average would be around $650K for 2024. You can compute the average for the second home (home B) as $300k /12* 4 = $100K since it is classified as a qualified home (not a rental) for all of 2024. This puts the total average balance for both homes at or below the $750K limit.
However, you don't have to calculate the average balances and apply Table 1 in Pub 936 at all (see the instructions for Table 1 at the bottom of Pub 936 page 10). Since home A had no balance for Jan - April and the interest for home B is applied on schedule E for May - Dec, the total balance for both mortgages didn't exceed $750K at any point in 2024. All the interest on home A plus the interest paid on home B for Jan - April is deductible as qualified residence interest on schedule A.
