A:Answer They are not designed to be stacked, so you will need to provide rubber foot cups or spacer to keep the top one level, high enough to allow clearance for the bottom door and keep it from sliding off when the door is operated. I would also caution against adding too much weight in the top one - while I doubt you could add enough weight to cause catastrophic failure, it could dent or ‘rack’ the housing of bottom unit, leading to inefficiencies or door problems.
