On our first cruise last year, my kids were in the age same group and this year they are now in different groups (10 and 12 as well). However, they both had totally different interests and rarely attended the same event at the same time. The one thing that they did do together was the nighttime event where they stayed out until 3am. They had a BLAST!!!!
If you don't know what it is, it is a late night party for kids in that age group. It isn't like the babysitting in that they stay in Camp Carnival all night (which...believe it or not...they wanted to do but hated....so just once), but they went around the ship and did different things. Then the directors drop the kids off at the rooms and groggy parents have to sign their kids back in. It was fun and something my kids talk about all the time!!!!
In any event, I personally would not put them in the same group this year just because they still are going to want to do different things. Last year, my son stayed in the pool practically all day and we tried to get him to go to some of the events, but he didn't want to. (We required some so that he could meet new people, which he did end up enjoying.) My daughter played in the pool occasionally, went to the events occasionally, hung out with friends or us occasionally, or changed her clothes a lot!
It sounds like you have 2 boys while I have one of each, but I also know that at age 12 they are starting to change and just aren't interested in hanging out with their younger siblings. I don't have the answer to your question, but I thought I would give you my feedback.
BTW, when are you going? We will be on Conquest on 7/30/06.