The idea is to give the impression that you have a CARD CASE HOLDER that holds two decks in their cases.  One is a blue deck and the other is a red deck.  You take the blue deck out to perform some tricks and then put it back in place.  Now you go on to do some other tricks with rope, coins, etc.   After finishing, the deck switch has already been made and can open the CARD CASE HOLDER to use the second deck which is  preset  for any trick(s) you want to perform.