Having two cards next to each other may well make the illusion of preventing the cards from working as quite a lot of readers will refuse to communicate with a

Having two cards next to each other may well make the illusion of preventing the cards from working as quite a lot of readers will refuse to communicate with a card if there is more than one in range (sensible for some applications). This does not mean, however, that they are not capable of doing so - the first thing a reader does is get all the cards in range to broadcast their UID - it then uses this UID to select a card to talk to. When I was cracking mifare cards (used as authentication in many buildings) I found that it was significantly quicker to crack several cards at the same time than to crack them individually - this is because the attack that I was using required demagnetising the card hundreds of times which takes a lot longer than any communication with the card. I could crack a single mifare card in 5 seconds, I could crack 5 in 6 seconds (and for most applications cracking one card is all you need - all cards use the same encryption key).