There are lots of ways cards can work well together though. Indeed the possibilities are almost limitless.
Definitely having multiple cards with the same institution that earn rewards in different ways but can be leveraged to earn rewards together can be very advantageous. For example in addition to signup bonuses and whatnot, if you have the Chase Freedom you can earn 5% category spend on things like groceries and gas. Then you can transfer those 5% rewards to the CSP for points toward travel - while also earning 2% rewards with the CSP on stuff like restaurant spending. So by having both cards you earn more by optimizing than you would with the exact same spend on only 1 of the cards. AMEX has a lot of ways to do this type of thing too.
However, as I said, there are lots of ways to make cards work well together. I have rewards cards from several different lenders so that I can always get at least 2% on everything I spend and often more. So even if one card only has a good gas or grocery bonus, I can get restaurant and utility bonus on a different card.
Other pairings/combos might be something like a good rewards card (or several) coupled with a low APR, non-rewards card if you think you'll ever need to carry a balance. OR simply adding a no-foreign transaction fee card to an already strong portfolio if you travel but didn't already have a no FTF card. OR even something really simple like having an AMEX or Discover that you use for daily spending but adding a Visa or Mastercard to the mix to ensure wider acceptance. OR perhaps getting a 0% intro APR card, or 0% balance transfer card if you need that, along with continuing to optimize your spend/reward on other cards.
There really are tons of ways to get benefits from using two or more cards together. It just depends on your needs, preferences, and priorities.
