Well a double backer is nice for different kinds of effects.
Like in a "all-backs routine" to hand out the double backer to a spectator (forcing it) and tell him to remember the card.
He will look at you with a "WTF?!?! impression on his face" you then look at your deck and realize that the deck has crashed.
You show,thanks to different sleights or moves,that the deck has no more face cards.
You ask the spectator to imagine this double back card has a face,which one would that be? 7 of spades?
After some mumbo jumbo all the cards having faces again and the spectators double backer is the named 7 of spades.
Just a quick effect you can do with a double backed card.I could go on and name some more but magicians will know how to use a double backed card.
A blank faced card is great to have too!
One of my opening effects with cards requires a blank face card for a reverse fan.
A laymen is not used to see a blank deck or even a single blank card.Keep that in mind!
I prefer having both a double backer and a blank face card instead of 2 of the same gaff card.
Edit :
You can easily make yourself blank face cards out of Aces and deuces.
Just take a bit of steel wool and rub/grind the pips and indexes of.Voilá you´ve got a blank face card
