I think that card clash is an important issue that needs to be solved but it has to be solved differently to this.
Firstly contactless readers need to start dealing with more than 1 NFC card in the field which is fully supported by the underlying standards. Despite only having 1 card in my wallet capable of making a contactless transaction, card readers reject my wallet because my work ID is in there. On the other hand, my work card readers successfully index the cards in my wallet and read from the work ID card to confirm me.
Now that fixes MY wallet. BUT in the situation you have multiple cards capable of making a contactless transaction in your wallet, @Mikeee is dead right - the reader will either see the wrong one, or see multiple and throw up.
What we need is a way to write a bit to the card to disable contactless on the card itself (not through the app/backend software). This would be as simple as holding your card to the back of your (android) phone and having it disable/enable contactless at the hardware level. I have no idea if the current MiFARE chips support this but if I were banks I’d be pushing them hard for this feature…
Just my $.02