There's a routine in "Front Row Sankey" called "Out of sight" that uses a double backer, but as it implies marking this cards with a sharpie, it can't be used later for any other effect, so it's up to you.
By the way, anyone looking for double backs, double fronts, etc should purchase "the art of card splitting" which explains very clearly how to make gaff cards, and teaches you some killer effects with them. (Dg also explains how to split cards in DGP1).