drcpn7 - it sounds like you're saying you don't want to do next summer's 7 day since it's the Mexico cruise from LA ? Though I haven't done a back to back, I've read postings on this board where others have done so and I don't recall any issues. I suspect you could even keep the same cabin if it's available on both cruises when you book.
The only thing I might question is the quick repeat of the same ports. But, I thought about this myself - I'm doing a semi-back to back in December - taking the Disney Wonder 3 day and then getting on the Sovereign of the Seas 4-day at Port Canaveral the next day. Nassau is a stop for both of these but I've decided to stay on the Disney Wonder on Nassau day and get off in Nassau when I'm there with the Sovereign since it's the longer of the 2 cruises.