each stateroom/home address can book 2 reservations for a single future cruise to get full incentives. You no longer can book a B2B onboard and get the benefits for both cruises. If you are doing a B2B cruise, you could book a future B2B by booking one leg during each leg of the current cruise.
I'm not sure, but I think you can still book a B2B on board and get the stateroom credit for both, but only one cruise would get the discount.