Welcome to cruisereviews! I would love to to a B2B some day. One other suggestion I have, based on the experience of close friends who have, is to remind your cabin steward at the end of the first cruise that you will be onboard for the next sailing. My friends (whose B2B was not on Carnival, btw) came back to their cabin to find that their personal items were removed from the bathroom when it was cleaned. Granted, this is a very rare likelihood, yet a gentle reminder can't hurt.
No, soda cards cannot be used for the second cruise. Yet, you can determine from your usage on the first cruise whether it's worth it to buy one on the second.
While I haven't been on a B2B, I have repeated itineraries on the same ship. We've always found something different to do on those subsequent visits, whether it was onboard (such as dining at a different time on the second than on the first, seeing different forms of entertainment) or onshore. You might even consider staying on board the second round if you revisit a port.
And, as it is, IMO, the passengers who make the cruise, there are definitely some activities worth repeating--such as karaoke and the Legends show, if you are into them.