I'm pretty sure you have the option to enter one, then the other after the balance is updated. If this isn't enough to cover your order, it will then prompt you to use another form of payment. I don't believe you need to combine the gift card balances in order to use both on the same order.
You may have to call customer service and place your order that way. I think they only have room for one giftcard at a time when you order online. You sould be able to combine them if you do a phone order or in store order. It's not like you are trying to redeem two promotions, or anything.
Hi 3y3shad0w, There is only space to enter one gift card online, but you can apply multiple gift cards to an online order by calling customer service at 1-877-737-4672 (1-877-737-4672). Representatives are available Mon-Fri 6am-9pm and Sat&Sun 8am-5pm PST. Please keep in mind, only one promotional gift card can be used per client. Hope that helps!