Just one more point.
Just because you are checked in for both cruises, don't assume that guest services knows that you are B2B. I've h had that situation before, and did NOT received the B2B letter in my cabin.
I always swing by guest services on the second or third day and verify my status and confirm that I will be getting the letter.
Have fun! The last night of the first cruise is satisfying knowing that none of the luggage in the hallway is yours!