Do we need 2 coke cards for two ?
Yes, each person needs their own card. You cannot share one. Frankly I don't think they are worth it unless you drink the equivalent of 4 or more 12 ounce sodas per day. Others think they are a great deal. We only buy the card for our son since it makes it convenient for him. Plus his card is cheaper.
Years ago, sodas during meal hours were included in the cruise price. But as the cruise lines searched for more revenue generation, away went the sodas. There was a time in between when there were no soda cards. It was all purchase by the drink. I forget which cruise line was first with the soda cards but it wasn't long before all the others followed suit. Frankly most of the bartenders and waiters hate them.