I have never done a back-to-back, but have spoken to quite a few folks that have.
It is true that if you stay on the same ship - you will get the same dining room menu's and the same shows for the second week. What some people do is pace themsleves, so they do some shows and some alternate dining during week 1, and then do other shows andother dining routines the 2nd week.
Personally, if I ever did a back-to-back, I'd change ships in either the same port, or one close by and start all over again. It is more of a pain - but I think staying on the same ship and reliving the same food and entertainment would be boring.
Best of all - book a cruise longer than a week - say anything from 10 to 14 days long. That way you stay on one ship, and all the ameneties onbaord are geared for the longer period of time. That is what we do. Never book just a 7 day cruise. Too much packing and traveling, and the trip goes too fast. We always do 10 days or longer.
Just my opinion.