There is just one name on the card, so only that person can use it. If one of you will be traveling without the other, it might make sense to get the second card. My DH and I don't typically purchase two.
We HAVE gotten two once when my FIL got a card, and he bought the second card and put it in my name, so that way my parents had a card, and so did my DH and I. That worked well, because my DH and I stayed a full week longer than my parents, so we did not have to buy our own full-priced card. I am not sure that is technically in the spirit of the second card policy, but we didn't lie to the CM, just asked for a second card in my name. We were prepared for the CM to deny that request.