Lets call them A & B. A is given a choice between a red-backed and blue-backed deck. Let us assume that he chooses the red-backed deck. The Magician shows him that the cards are regular and well mixed, then hands him a dagger and asks him to touch any card with the point of the dagger, as they are spread face downwards. The card chosen is taken from the pack without being looked at or shown to anyone and is placed by A in his pocket.
Turning to B, the Magician shows that the blue pack is also perfectly ordinary, he shuffles it and wraps it in a piece of newspaper. He hands the dagger to B and asks him to thrust it into the wrapped pack. B does this. The parcel is torn open and the card at the point at which the dagger entered the pack is removed. Let us assume that this card happens to be the JH. A removes from his packet the card that he freely chose. It is also the JH!!
Includes 2 decks--Does not include dagger.