Cruising back to back does require you to get off and go through customs, but the ship will arrange for those staying the next week to all go at once.
Essentially they will tell you to meet in a certain lounge around 9am and the dozen or so will go together after the lines are gone. Expect it to take 15-30mins, they usually have coffee/pastries while your waiting to go. Then you turn around and get right back on board, usually it's about 11 before the next passengers come on board so the ship is yours. Most of the ship will be closed though so don't expect to go to breakfast afterwards, just hit the pool or something.
Also you don't need to stay on ship, as your in Miami I'd suggest getting off at 7 with the walk off passengers and heading to the beach for the day. You will need to be back on board by 3pm.