In our experience, this is not true. We have done this multiple times. We usually cruise with friends of ours who are not Diamond yet. We book my wife and I in separate cabins and book one of them with us. Once on the ship we go to Guest Services and explain who will be in which cabin and how their on board expenses will get paid. Guest Services makes new cards and assigns the proper expense accounts. So, my recommendation is, go to Guest Services, expalin to them (nicely) what is going on and ask them to issue new cards.
We actually did this at check-in at the pier when we did our Alaska cruise several years ago.