How to share multiples rooms as one room!

I was thinking the same thing Bry when I discovered this, but yes unfortunately we cant delete individual floor tiles :(
But you can section off a area with a fence/half wall using this method and not create a room. This helped me tonight while I was creating a fake crib in a nursery I was building.
I used a fence to make 3 sides of the crib and the head of the crib was against a wall. As soon as I made the crib like that it created a room and the roof popped into the nursery. So I used this technique and had no problems :)