Someone I know was delighted to get his over-sixties bus pass. He made great use of it but was very disappointed to discover that the bus readers were debiting funds from other contactless cards in his wallet when he waved it at the gizmo.
Thats another urban myth. Only one transaction can occur no matter how long you hold a card. The reading is limited by NFC (near field communication) which is around 4 to 5 cm. If more than one card is tapped on the reader at the same time, there is a conflict and nothing will go thru. You can try it by placing your wallet full of cards against a Tube reader or a supermarket proximity reader.
People tend to get confused when they read their statement and find out that their debit card has a contactless charge when they usually use their credit card or another bank's card. They then wrongly assume that it pinged them twice. It just that they have inadvertently pulled out the wrong card instead of the usual card.
Here is something folks can do to see if these claims have any merit. Go to one of the supermarkets late at night on a slow day and try every possibility for a bar of chocolate i.e. press multiple cards on the reader at the same time, place the wallet full of cards , leave the card against the reader for long as you like, against the reader, tap the card multiple times etc.
NFC was created just for this and bluetooth technology which has much longer range was not allowed.