Sorry I wasn't clearer in my first post. The reason you use two knives to make one, is to ensure that the blade is identical on both sides. Imagine the knife with both blades open: it would be in a "Z" or "S" shape. The knife itself isn't really gaffed. If Daytona sells something like that, then they're the only dealer I know of who does.
The Kennedy knife has one blade, and the other side is a "blade looking" file/screwdriver appendage the mage has to pass off as a blade when the card is stabbed. Scotty York's devising allows for greater spectator scruitiny in a close-up performance.