I believe he is trying to add an extra Fuselage like the twin mustang model.
Rc Man100
The Flight Physics it just for adjust the FLIGHT PHYSICS of a model. If you added a extra fuselage to one wing, the model will fly as if it had that extra drag and weight of another fuselage. It will not change the appearance of the 3d Model in any way.
You would have to model a new 3D model with that extra fuselage your trying to add, give it a name like ~CS_FUSE2, Add the extra Fuselage in the Flight Physics editor, Pick the ~CS_FUSE2 from the drop down list of parts you want RF to try to mimic for the shape of the new part, then adjust it’s new weight and drag factors.