Peter, We are doing back to backs on Enchantment for Christmas and New Years. It has been my experience that back to backs should be booked directly through RCCL or your TA. The reason being is that they are coded so that the cruise line knows in advance that they are back to back. RCL has a special back to back department that works with TA's. They only issue is that the B2B code does not allow the cruises to show up in your online account
As for cabins it takes awhile to find the same cabin on both sailings. The agent we used talked to the B2B department and I think that they may have more options than what you see on the website. We booked a year in advance in order to get the same suites and it almost didn't happen.