I've gotten quite a few of these over the past year, and on a recent trip to Miami, I used two of them to pay for my entire trip, so don't think it only allows one per booking... it is basically like applying a credit of that amount to the ticket, and if you need more, you apply more...

I did purchase tickets for my wife and myself, so not sure if buying for two people is why I was able to use two certificates, but just my experience...