A room is already double-sided if it belongs to a level with an elevation > 0.
As in the attached example, you could:
- create a new level
- draw your shape with walls in the elevated level
- add a room that will cover the entire surface of the walls by double-clicking on the walls instead of double-clicking inside the walls
- copy the room
- add a new level
- paste the copied room and hide its ceiling
- choose Edit > Select all at all levels menu item
- finally, export the selection at OBJ format or use CopyAsNewFurniture plugin

Et voilà ! A star is born [;)]