You will need to get off the ship, with others that are back to back, briefly to go through customs, then will be let back on the ship. You are usually taken to a separate room away from those disembarking the ship.
if you booked the same cabin number for both cruises, you will not need to pack. If you have different cabin numbers for each cruise, you will need to pack, but your luggage should be moved to your new cabin for you.