I carry discover and a MasterCard with my Amex. Worst case scenario, I have my Visa debit that I carry for emergency cash. Unless their terminal is down, they're getting paid.
Also, it's extremely unlikely where I live. I've maybe ran into 3 or so places that don't take amex. It could happen though. Not too long ago both the visa and MasterCard networks crashed within a month of each other.
It doesn't hurt to have an extra Visa/MC with you. Usually, if the Amex doesn't go through it's because they don't accept it or some sort of hold was put on your card (which is uncommon unless fraud is suspected {unlikely} or you spent well beyond your limit).
I imagine a large part of this community walking around with 10+ cards for every CB/point optimization scenario but realistically, if your lifestyle isn't one of min/maxing rewards, I would say 2 Visa/MC and 1 Amex should offer sufficient redundancies in most scenarios.
Must garden until 2/1/2022 to hit my goal AAOA. 
I always carry a backup for Amex. Right now it's a Citi DoubleCash. If that gets declined I can use my debit card... or one of my other cards I might happen to be carrying. My Affinity has a quarterly bonus for clothing and furniture stores, so if I happen to be carrying that, it also serves as a backup to my others.
Apple Pay for me as well for points maximizing. I recently moved to a slim wallet and in addition to a few of my Amex cards, I carry CSR and Citi Premier. The only times I normally run into "no Amex" is when paying for parking at a meter or kiosk, and then in addition to not accepting Amex they also usually don't physically accept CSR due to its thickness and being rigid.
@Anonymous wrote:Hi, everyone.
This is a question for only those who carry a AMEX (any AMEX) card and one Bankcard (either Visa or Mastercard).
The scenario is that you go into a store and try to pay with the Visa/Mastercard, and for one reason or other, it is rejected. Then, you try to use AMEX as a backup, but the store does not accept AMEX. So, you are left with no card to use.
Would you want to carry a second Visa/Mastercard as a backup, or is this scenario so unlikely, that you would just stick with the two original cards?
Have a great day, and stay healthy !!
Personally, I think it is unlikely, but I'd carry a backup anyway. The only time I've been declined at a store was back in like 2005ish when I was out of state and bought an xbox at a Best Buy. I went back the second day to buy a game and my card declined. The cashier called the credit company, who then requested to talk to me, and they unlocked my card (was a fraud lock). I've been fortunate and never had to worry about a card declining for some other reason, so a fraud alert might be the only issue. If it every happens to me again in the future, it could be cleared up with a (hopefully brief) phone call, or I'll swipe a backup card. It couldn't hurt to carry another one.
Additionally, I was declined online about 2 years ago buying tires. They charged a penny before the actual charge, so the purchase attempt was declined for fraud. I think it was with BoA, and they called me like half an hour later (already used a different card). That's when they told me why it declined and then unlocked it for me and made a note in case I used it again on that website.
