Mamie,
I just got back from a back to back cruise in January and we did not have to disembark and embark again. You could either go off in the port and board again like a regular port or you could stay on the boat.
I personally enjoy back to back cruises. I recommend staying in the same stateroom for both segments, otherwise you will have to pack and unpack, which is no fun.
I have a personal blog where I covered this topic in depth here if you would like to check it out. I also have done a lot of research into various back to back itineraries offered in 2021 and have laid them out here because I know how hard it can be to find these specific cruises.
I hope these tips help!