I'm not sure about how RCI does the Back to Back cruises. I think Sonny recently did one and will know. I would imagine you should have both passes just in case. On Carnival, they escort you off the ship to go through Customs and then back on so you don't have to wait in the terminal. Then they re-check you in at the Purser's Desk before the new guests come aboard. Sorry, that wasn't much help. Anyway, Congrats on your first Back to Back cruise!