You state that you are ending one lease (with the two tenants) and entering into another (with the remaining tenant).
When you receive prepayment of the last month's rent on a lease, then... that is the last month's rent. It's not prorated -- its simply applied as the full last month's rent. If you overcharged rent due to a discrepancy between the date rent is due and the date the rental period starts, you may refund the amount that you overcharged.

