Each code is unique so the code will only work one time only. Did you put them both in the same order? I think it might work if you put them in separate order or it might be just one per person, the bonus box description thing says limit one per customer order.
"Order "Final Fantasy XIV: A Realm Reborn" and receive early access to the game. We will send you a promotional code via email within two days after your order with redemption instructions. Offer valid when shipped and sold by Amazon.com. This offer will be extended to all existing pre-orders. Limited to one per customer order. Amazon reserves the right to change or terminate this promotion at any time. Here's how (restrictions apply) "
note : Limited to one per customer order. and "Offer good while supplies last." from the "here's how" info.
Shouldn't have been set up like that in the first place. We had the same issue (as we didn't read the "one per order" BS until it was too late). However, when the bonus is early access to a game, and you are ordering TWO copies of that game, it should be obvious that there will need to be two codes. I will admit to not reading the fine print, but that doesn't change the fact that this was a horrible move for Amazon, which normally has a history of great customer service.