To clarify, it is not double points, necessarily, but it is more points. In your case, for example, sailing out of Miami for 14 nights in YC, you would get 2000 points as one cruise (based on the cruise being over nine nights). As two cruises in a B2B, you get1500 points for each week for a total of 3000 points. Still significant, but not double.
To us, the biggest advantage is we can have me as primary on one week and DW as primary on the other. That allows us to more equitably distribute the points that go to the primary passenger. No advantage in having two Diamonds on a cruise together, but if she is planning a girls' cruise, at least the Diamond benefit goes with her.