I don't know about the visas, but on a B2B I did on Liberty Of The Seas a year ago many of the B2B guests were from Germany and France, and they had no problems from what I could tell. All B2B guests are considered 'in transit' and thus not re-entering the U.S. on the turnaround day between cruises. I recommend you have your agent check for you, or call RCI directly. I was just an observer on those cruises and not involved in the processing of guests. I just looked at all the documents they gave me at our "consecutive cruiser" meeting on the day before our first cruise ended, and there is no mention of visas.
Everyone does have to leave the ship as a group and do a walk-through check with Customs and Immigration officers. We were told to bring a completed customs form, our passports, and an 'in transit' card the ship gave us. The officers barely glanced at everything and waved us past them and back we went to the ship. They didn't even take the customs declarations.