We did a B2B last cruise. We went to Jungle Island in Miami after the ship pulled in. They gave us each a card in our rooms the night before that said we were on a B2B and when we came back to the port we showed those cards, skipped the lines and went right to the next available counter and got our new Sail and Sign Cards. They told us that if we stayed on board, they would have shuffled us through to get our new cards and right back on board. We wanted to visit a few places in Miami though and it all worked out smoothly. It only took us about 10 min to get our new cards when we returned to the port. We did our laundry Friday the afternoon before returning to Miami as it was a sea day.
Previous Carnival sailings:
Carnival Sensation 2013
Carnival Liberty 2013 (Eastern)
Carnival Liberty 2013 (Western)
Carnival Victory 2012
Carnival Triumph 2001