They'll check you in for the next cruise while you are on the first one. On debarkation day for the first cruise they'll have B2B people meet someplace on the ship (they'll tell you), if they haven't already issued your new cruise card they'll do it at this meeting (the card for the first cruise is not used for both legs), and then if the local Customs officials require it someone from the ship will escort you off and you do a walk-thru with Customs and then reboard the ship. You will bring your passport, a Customs declaration for the first cruise, and the ship gives you a 'in transit' card which you show Customs. That's all. You really just do a walk through where they hardly look close and wave you by. Detailed instructions are provided before the first cruise ends. I don't know about Carnival but on RCI we had a meeting on the last afternoon of the first cruise where they handed out our new cards (so yes, I had two cruise cards for a day but only one worked) and provided a briefing.
You don't have to pack up and leave and then check-in again like a brand new guest. If you are in the same cabin for both cruises you just leave everything where it is. If you are moving to a different cabin the cabin staff will coordinate the move and they will carry your clothes and stuff to the new cabin for you.
I have done several back-to-back cruises on Carnival. They give you a letter in your stateroom with instructions on where to meet on the morning of debarkation. For us it was like 9:30 am or maybe 10:00. We were escorted off of the ship by a staff member from Guest relations and then taken directly to the next available customs agent (bypassing the hundreds of guests in line) who just waved us through. Then, when we returned to the ship we were issued our new sea pass cards and a new photo was taken.
The whole process took less than 15 minutes and it was really fun watching everyone leave the ship, knowing that we got to stay for another cruise :-)
No they did not, although, we did get a bottle of champagne each time.
I can honestly say that cruising back-2-back is my favorite way to travel. I am looking into doing it again over the Christmas break this winter but I have to work it out with my oldest daughter's schedule.
I agree with you, Dave...It's almost like, when everyone else is lining up to get off of the ship, that you have a secret that no one else knows. Many of them were looking at us like there was something wrong with us because we didn't have our luggage or handbags or anything else with us. I feel bad saying it, but I really enjoy hanging out and watching the debarkation process knowing that I have another cruise awaiting me.
Of course, the more you gloat (on the inside, of course) the harder it is when you have to get off. The only saving grace is that after two cruises, the thought of sleeping in my own bed and showering in my own bathroom is at least somewhat appealing to me.