I am thinking about having 2 gaff cards in the deck. The first gaff card would be a double back, I think this would look good as the first card on the uncut sheet. Here are 2 possibilities for the second gaff card, which do you think would be best?
Let me ask you an even better question: how much use do you think a limited-edition deck such as this will see i the hands of a performing magician? If I'm not mistaken, the answer is "not much," so perhaps something else is in order. In fact, if you print with a company such as Expert or Legends, you won't HAVE extra cards - their uncuts have only 54 cards on them, so you'd have to sacrifice Jokers to make gaffs.
If you have extras, consider some combination of the ideas I'm going to list - be it one and another or two of the same:
1) an advertising card, listing your company and mentioning your projects both current and future.
2) an advertising card that some lucky backer will pay a top-dollar tier for, in order to advertise his or her company.
3) a "cut card" of a solid color on both sides, different enough from the rest of the deck that it stands out like a sore thumb (poker players use cut cards a lot).
4) a card listing general poker rules on one side, hand ranks on the other
5) a "replacement card" - a card with blanks in the right places, to be used as a substitute in the event the deck winds up with a missing or damaged card after some use. Pull out the damaged card, drop in the substitute, and figure out some means by which the card is easily identifiable. Two good examples of this type of card come from the Hornets by Circle City Card Co. and the Misc. Goods deck from Pedale Design.
Re the ad cards, you have the option of keeping a side for yourself and selling the other side to someone else - or selling each side to a different company. Find the right price point and that tier alone can give your project a serious shot in the arm.