You can have a spectator select a card, and have the back signed, and then do the same to another spectator, and you can fuse the two signed backs into one card with the double backer.
Sneak it into an Ambitious Card routine and combine it with the Tilt move and you have Vernon's "Trick that Fooled Houdini."
My favorite double backer effect is John Bannon's "Tattoo You," although you need a red/blue double backer. It can be found in his book "Smoke and Mirrors."