R.L.
Hi A.,
While I'm not the mother of twins...I am a twin. My parents let me and my sister share everything for way too long. Luckily we were both girls. We even had to share a full size bed. At the age your children are this may be fine. But since you're concidering building a house, also make sure to allow them individual space. My sister and I fought constantly when we were younger. I think it was because we were refered and treated like "The Twins". No individuality! Now we are extremely close. We are also extremely different.
As for my children...I have a boy and a girl that are two years apart. And while they each have different rooms and different toys, I always find them together in one room or the other. In fact on weekends they usually fall asleep watching TV in one room or another. So I feel that by allowing them thier individuality...it has made them closer.
Hope my babbling helps.