I've done a number of back to back cruises. Started out just doing them on the same ship, with alternating itineraries.
But, I did find the repetition of the menus and entertainment to be a bit of a drawback. So, we started changing ships, and cruise lines, for the second leg. Once we even changed departure cities, and went from FLL to Miami on tunover day.
Our preference is to go through the bit of hassle to change ships.
Howeever, if you choose to B2B on the same ship, if you book the same cabin, you don't have to move your belongings, and you only have to leave the ship long enough to check through immigration. If you choose to, you can leave the ship for the day, and treat it as you would a port visit.