Try 3M Super 77 spray adhesive. I have tried just about everything and this stuff really is extra tacky and holds the best, especially when making acrobatic cards.
Also, if you really want to get the back-to-back gaffed card right, you should split the cards first so you don't have an extra thick card. If you are splitting the cards, I also recommend that you use a hand roller to roll out any bend caused by the splitting, air bubbles, or excess glue. Then press the card overnight so it is perfectly flat. I sandwich cards between two welding glass faceplates and four powerful spring clamps. By the time it is done, the gaff should feel and handle just like any other card.
Regards and good luck.