Hello. I’ve read through many question and answers on this subject but can’t really get an answer. I am stacking one possibly two cake dummies on top of a real cake. I found tips on how to attach the dummy cakes to each other but I’m unsure about the dummy cake that sits on the real cake. Do I cover the bottom of the dummy cake with buttercream? Just leave it foam or add a board? The real cake is buttercream the dummy cakes are fondant. Do I add dowels to my real cake? Wood or other? Thanks in advance. 

I would dowel the cake a little because fondant can get heavy esp if there's two tiers above there-- I use plastic dowel -- I would put a cardboard circle under the bottom dummy to keep things sanitary on the real cake -- you can attach the cardboard to the dummy with buttercream -- no buttercream underneath the cardboard circle though toward the cake  -- and nothing needed between the dummies of course ==

best to you

Yes as K8 said be sure to put a cake circle under that dummy.  You don't want those foam crumbs getting into the b'cream cake.  I also agree w/her to put 3 dowels in the cake to support the weight since you are covering them w/fondant,.  If they were b'crem I probably would not dowel :) 

I would use 5 dowels for stability esp for two tiers above