Someone will post shortly in more detail the process.
A tip to save a few dollars is to sail on the 3 night cruise first, then the 4 night cruise. The reason is parking at Port Canaveral parking garage, you will be charged for the first cruise (the 3 night cruise)only.
Will try and answer your questions:
Do they make you pack up and ship out for a short amount of time? If you have the same cabin, then no, you do not pack, but you do have to leave the ship for customs.
Would they allow you to leave your bags on board? Yes, your bags stay on board. Of course take items like your passport and such with you.
Would they even know you did it, provided they don't see it in the booking process? DCL will know. Your Key to the World card is scanned as you disembark.
Search this board for B2B or back to back for tips.
B2B are fun.
Wife and I did our first B2B last year on the Dream and we have our second B2B set for the end of October on the Fantasy.
Good Luck and I hope this helps,
Glen