In my experience:
- Business accounts assign a unique number to each card. Any issuer, any network: Amex, MasterCard or VISA.
- Personal accounts issued by American Express assign a unique number to each card.
- Personal accounts issued by other US based banks, even if Amex-network, typically assign the name number to all cards on the same account.
However, if a card is swiped the issuer can determine which card was used, even if the embossed number is the same, and there should be no problem with cards being used in different shops, cities or countries simultaneously.
Ah ok thats great. Thanks for your help guys. Thought I'd better check haha, first time I've been with an issuer that uses same card number.