What part are you waiting for an answer to?
Is there a reason, that by my calculations, it is less expensive to book the two parts of a two part combo cruise separately rather than booking it as one cruise? Do you receive any unspecified benefit for booking a combo? Is there a downside to booking both parts separately such as having to disembark and then check in again?
I have heard that people sometimes have to disembark (but not pack) and then basically go through customs again, but that is probably solely dependent of what country you are in (U.S.).
These lux lines, especially Seabourn, market different combinations of cruises as different singular cruises, so the savings is built into the pricing scheme and I don't think you are officially booking a back to back much of the time.
For example, the "brochure" will list a 10-day cruise, a 20-day cruise and a 31-day cruise all starting on the same date and same ship. They are combining cruises for you, and the price goes down accordingly.
This is why lux lines rarely repeat any itinerary, they know a lot of people just want to stay on the vessel for more than one cruise.