Coincidence Mene-Tekel

FOR this effect, which Mr. Gravatt considers one of the best of
the tricks of its type extant, you require a Mene-Tekel pack having red backs, and an ordinary pack with blue backs.
Show the red-backed cards all different, have a card selected, and bring the duplicate to the top. Take the chosen card and place it casually on the top of the pack, then lift the top two as one, showing the face of the lower one, and place the two, as one card, in a glass tumbler with the backs to the audience. Both cards being the same no suspicion can be attached to this move.
Have the unprepared blue pack shuffled, any card selected but not looked at, and place it in the glass behind the other two cards so that its back is nearest the audience. In your patter lay great stress on the fact that the two cards were freely selected from different packs. Turn the glass around, remove the front card, leaving the duplicate facing the audience. The two cards are the same. Show the red back of the card in your hand, then turn the glass and show the blue back of the other.