having 2 discounts

Hi my client wants to add a second feature to their ecommerce website. So on the checkout page (where customers will enter their billing/delivery address they will also have the option to enter a "gift voucher code" and/or "special offer coupon code). Currently they website exists with the gift voucher code they then gets calculated by paypal etc. but I just wondered how I can integrate a coupon system based on a percentage discount....also logically the coupon percentage discount would need to be calculated before the "gift voucher" discount is applied to the order. since the gift voucher is a means of payment (since it would have been previously ordered and paid for). What would be the best way to go about having 2 different types of discounts using ecart (so the gift voucher just reduces the total order cost and the coupon take a percentage of the total items (subtotal).

Chris