If two NFC cards are in my wallet, what prevents both from being charged when I tap for payment?

Very interesting question. I can answer it from a project on which I am working on where these very specific questions were asked.

Most apps are designed so that either electronically or manually, a "bill" is generated and pushed onto the NFC terminal onto which a payment can be made by an NFC enabled wallet. When you bring a wallet that has more than one NFC enabled, direct payment card on it, the closest one to establish contact with the NFC terminal for payment would be the one that would be selected to pay up. The 2nd NFC card will not have a bill waiting to be paid. Hence, the 2nd NFC card would nee any pending "bill" (or charge) awaiting at the NFC terminal.

Most NFC applications are also designed to that a user prompt (or authorization) is required on the phone, which prevents from people walking close by in proximity to an NFC terminal having their cards charged.

So essentially there are security features on both ends:

• The terminal needs to be prompted to accept a bill (charge). Absence of which, if you bring NFC card close to the NFC enabled devices nothing will happen as there is no bill to be paid.
• To prevent automated skimming or proximity skimming of NFC enabled wallets, most NFC phones, will require a user intervention on the screen before the NFC payment can be made.