Simple interest bi-weekly mortgage is a payment plan paid paid once every two weeks. Here, the payments made by the borrower are applied right away as they are received toward the remaining principal balance of the mortgage. What differentiates the simple interest bi-weekly mortgage from the traditional bi-weekly plan is that the payment applied at once. In the traditional bi-weekly plan, the first payment is held until the second payment is made before it is applied toward the mortgage’s remaining principal.