Sandy, great point. I did do that once with when we brought my oldest along; we had a suite and got her a balcony. With just the 4 of us, we keep the kids with us. Sometimes they're almost free. I never thought about a cheap inside room across the hall.
They're gone most of the time anyway, don't take up a lot of space, and there are some great activities on the ship that guarantee both kids will be absent at a certain time...