I've used a rewards card to pay part of my MFs (didn't have enough on the card to pay the entire amount due). They applied the rewards card first and then I paid the rest with a credit card. If they can do that I expect you can use multiple gift cards towards your MFs.
Yes, you can pay with multiple gift cards. If you have a bunch of cards, you can photo copy them and fax them instead of sitting on the phone giving them all of the numbers.