Very rarely you only get the airfare reduced on the second cruis.
It pays to stay with the same lines for the best deals on repeat cruises. We are now on our fifteenth with Rcl and you do get reduction on balcony and suites if you Join their Crown and Anchor Society. which you can also use for Celebrity, but cannot combine the points. The best thing about their diamond memebership after 10 cruise points is acces to their conceirge club each evening between 5-8.30 where all your drinks are free and big saving over a 12/15 night cruise, which is really a discount off a cruise