We have 2 cards on the one account, but each has a different number. This happened about 3 years ago, but previous to that, both cards had the same number. Just prior to our first visit to Britain, in 1993, our bank manager suggested we take 2 separate cards, each on a different account, so if one was lost or stolen, we could use the other. With 2 cards having the same number, if one was reported lost or stolen, the account would be frozen, and both cards would be de-activated.
Now, with each card (on the same account) having a different number, if one is lost, the other can still be used. We have just been issued with new cards, following the expiration of the old ones, and the numbers are the same, with the exception of the CVC numbers on the signature panel. The PINs remain the same, too. Any automatic debits to the cards are unaffected.
Peter.