After adding a Room Combine Processor to the Processing Map and configuring the number of rooms, you then need to configure the actual layout of the rooms as well as which walls are movable.
You can extend the movable wall across multiple rooms, if necessary. For example, in the following layout, let's say that the wall dividing Room A from Rooms B and C will always be closed or open all the way. It will never be open just between Room B and Room A or between Room C and Room A. When you initially drag the movable icon to the rooms, the walls look like this:
It looks as if you have to create two movable walls for Room A—one between Room A and Room B and one between Room A and Room C. But that's not what you want. Instead, create just one movable wall (either between Room A and B, or between Room A and Room C), and then expand the wall across Room A:
As you specify movable walls, you can see all the resulting combinations in the Room Combinations box on the lower left:
The next step is to configure control of the movable walls.