To separate one room into two or create a partition wall, open the Rooms section of the catalogue and choose the tool with the pencil:
There are several ways you can use that tool:
To draw a room from scratch like you used to do before. Create the shape of a room by tapping in each corner of that room. Close the area by tapping in the first corner to finish the room.
Divide one room into two without drawing another room. Check this out:
Create a partition wall and drag it wherever you need. Double-click at the end of the wall to create it:
Note: this tool is only available on the Web at the moment.