I'm doing a back to back in December on the Jewel.
You don't have to leave the ship unless you want to . You will have to get a new sea pass card, but there should be instructions on what you will have to do. I was told you don't have to actually get off the ship, but a representative will take you to get your new sea pass card with a new picture. You don't have to wait on line again.
I assume you may have to settle your sea pass bill and start all over again for the second part.
They treat each cruise as a separate cruise. I did not get a discount for booking two cruises at once.
The only difference is you can book the same room and only have to pack and unpack once.
RCL doesn't have any self laundry on board, but they do have a laundry service for a fee. I don't know of any place at the Ft. Lauderdale port where there's a laundry facility. You would have to take a taxi into Ft. Lauderdale to do that. It would be cheaper to do it on RCL.
I'm looking forward to my first back to back. Too bad our friends will not be joining us as well.