Marco's Comments:
This is a trick based on topological oddity. It has been reprinted in "Secrets".
Almost more of a topological puzzle than an effect, two large playing cards are shown glued together back to back with a window cut through the center of both, with a few deft folds the magician hands the cards back to the spectator now glued together front to front, the magician folds the cards again to return them to their back to back state, all can be examined.