We did a B-2-B last year on the Wind Surf, Nice to Barcelona, Barcelona to Lisbon. It was wondeful.
We agree that one of the points is to get the same cabin. What a wonderful feeling it is to watch everyone put their suitcases in the hall, say goodbye in the morning, then leave, while you remain as a passenger. We had a great city, Barcelona, to explore, got right back onto the ship (but still had the champagne) when we wanted to do so, and had a great lunch before nex PAX were allowed to board.
Whether the memu changes depends on the line. I believe that Windstar used a 10 day menu, so there was some repetition but not 100%. The on deck barbecue, however, remained the same.
Yes, you do tip twice. As for entertainment, if it's standard entertainment it will repeat. For Radisson, if you take two consecutive cruises, at least one of them is likely to have a theme, making the entertainment different for the two segments.
All in all, if the itineraries for the two legs are different, do try a B2B! We do have a caveat, however. The age group on 14 day cruises tends to be older, and the cruise more sedate, than on a 7 day cruise. Other factors, such as itinerary (Alaska has an older average age than the Caribbean) and the season (school vacations obviously attract more kids than cruises during school). This factor can cut either way, depending on your age and the degree of activity you like: just bear it in mind.