Some have had success, however, once on board they adjust the price of the first diner to the on-board pricing. An advantage to this process is that you have reservations of the night and time.
On my August sail on Oasis, there was a party of 7 for which I had reservations for Chop's and Giovanni's however, when I tried to get myself to dine with my party and using the BOGO on the 2nd night for Chop's, I was told that they could not accommodate an 8th diner. All tables were pre-set. I did get the choice of changing the dining date to a different night other than 1 o 2.
I have the same dilemma for my upcoming family Anthem Easter sail. Should I wait till I get on board to make dining arrangements and run into the same hassle or pre-reserve Chop's for 2 (me +1) in my planner and have it adjusted for BOGO.