There are 2 situations:

1 - Exterior walls => in Punch, the exterior walls must be an enclosed perimeter. In this enclosed perimeter, the walls share the thickness.

2 - Interior walls => if you created an enclosed perimeter (exterior walls), you can add the interior walls using different thickness, as you can see in this image


Important concepts:

- if you have two walls and you join them using the arrows of your keyboard, you are connecting the walls => this means that the walls are together but they are independent => they don't share the properties and work as 2 separated walls.

- if you have two walls and you join them using the mouse, you are merging the walls => this means that the walls are fused => they share the properties (including thickness) and they work as only one wall.

 

Note that exterior walls should be fused.