The best way to turn one room into two would be to squeeze the first room and add a new one right next to it.

To squeeze a big room, you'll need to drag one of the walls down.

To draw a new room, you will need to use a drawing tool or insert a room of a predefined form.

This way, you now have two separate rooms, and you can continue styling.

Check out that short video to get a better understanding of the process: