I recently bought some new decks and many of them came with Double Backed cards and a duplicate. While I have a number of effects to use the duplicate, I have never really used a double back card for any effects.

Welcoming any recommendations for:
-loading a double backed card during a routine
-routines or effects using double backed cards

Afinemesh from Max Maven's wonderful book, Focus.
Exactly the reason I chose that name for a handle. . .

Harapan Ong's book Principia has a small section with the 4 extra cards you get in so many deck these days. 2 jokers, 1 blank face and 1 double backer. Like little packet tricks! They're cool. He's got a new book called The Four Treasures, which is some gaffed stuff, but I dunno if that's with regular gaffs ones or not.

Encyclopedia of Card Tricks has some info as well. Personally I use them for things like Darwin Ortiz' Back Off routine and for Anniversary Waltz.

There's so much stuff out there, and it feels like theres's a good book specifically for double backs, but I can't remember...

I would add Capitulating Queens to this list as it is one of my go-to packet tricks.

Yes !!!!

I've used one on occasions for the "perfect force." Looks like a single cut without a second "cut deeper"
Phil

There are lots of effects where you use a DB as a dummy then do a double turnover. The DB just looks like the next card in the face-down deck.
Phil's example above (Balducci / Cut Deeper force is an example.

Here is another (Card to Wallet) -
https://www.youtube.com/watch?v=mRN5D0hxCK4

There are more sophisticated versions of CTW (usually requiring you to palm a card, and have a special wallet). I love this version because it is EASY - and cheeky.


[You can also buy red/blue DB's, which can be used in some amazing effects with cards from two decks of different colours.
John Bannon has a great one called Tattoo You. The effect is that the spectator signs the face of a red card, then the signature is magically transferred to a matching blue card.]

It's quite easy to gaff a card box to secretly hold an extra card. You can use the deck as usual, then put away in the case, then take them out again for "one last trick" bringing out the DB with the rest of the deck. You can glue a piece of cardboard inside.
OR
A lot of card boxes have the image of the back pattern on the case itself. (eg some bicycle decks have these "old style" cases, but not the "standard" bicycle decks). Keep the cellophane on the deck, and slide the extra card inside it. The card matches the box so is perfectly camouflaged.

I use it in my routine for ambitious card. Definitely makes it a head scratching routine.