I've done a B2B in same cabin, B2B changing cabins and even as the Dutchmen mentioned B2B on different ships. They all have advantages and disadvantages so you need to have an idea of what you want your experience to be by doing a B2B. I think the Oasis would probably be a good choice for a B2B as Ready2 did and at least get a ship that has alternating itineraries. You usually get a gift basket on the 2nd leg of the B2B and if you arrange it you can pick your dining table as well. Enjoy it if you do it.