Welcome to the wonderful world of cruising. Your boys should be fine with the sofa. I usually take the inside cabin with bunk beds and my boys 12&16 are fine with that. The kids are so busy in the day and evening by the time it is bedtime the kids are so tired they don't have an issue sleeping on a bunk bed or sofa.
The plus side to having two rooms is you have two bathrooms which comes in handy when everyone is trying to get ready for dinner.
I did have two rooms once but I did not care for it I found myself going back and forth between two rooms all the time even though they are right next door. It seems like everytime I was looking for something it was in the other room. Buy maybe I just did not organize my unpacking well enough.
Enjoy your cruise! Your boys will love the liberty!