Hope this isn't too late - just joined the forum. Here's what happened when we did it... First, when you book you two cruises - make sure they are linked together at Carnival either by a Carnival representative or travel agent. When we booked our back-to-back, the rep forgot to do this. It's important. They look at it as if it is two distinct cruises and not a back to back. Try and book the same cabin. We were able to do that otherwise you will have to move your stuff on debarkation/embarkation day! Since having fixed the "linking" issue and booking the same cabin, this is how the rest of the process went - although - we have heard many other situations that have happened to others.
We notified guest services the day before debarkation day that we were doing a back-to-back and they told us what would take place. This was the Elation out of Jacksonville. They told us to meet at guest services shortly before the last "zones" got off the ship. We stayed there and they gave us new sail/sign cards, took our picture against a back-to-back backdrop in the Atrium, gave us Mimosas (!), some playing cards (just as a gift) and we were done. Some people say that you MUST get off the ship (set foot on American soil), some say you will be escorted from the ship through the terminal and right back on. Neither of these happened to us. We just stayed on board. We were the ONLY back-to-backers on those cruises and we really had the run of the ship. They made us feel so special.
It IS kinda strange (eerie) though being the only passengers for about an hour or so before the new passengers come on board. This big 'ole empty ship, a couple of crew members getting from one place to another and crew announcements that you probably aren't supposed to know about (tehehehee)....
If given the opportunity, we'll do it again!