I am going to make my first stacked cake this weekend. It will be 2 tiers, 6" on a 9". When I put the 6" one on top of the 9"...am I supposed to put something in between like those cardboard plates or do I just put the cake on top? Also, should I decorate the separate tiers first or after I stack them?
first: WELCOME!!!!!!! so glad you joined us and our favorite addiction!!! (read the WARNING link in upper left corner)
a tutorial to help w/ stacking: how to stack
Better safe than sorry, so DO put cardboard (covered w/ waxpaper or foil or plastic warp so it doesn't get soggy) under the top tier. Do dowel.
can do decorations either way -- depends upon complexity of design. just don't do bottom border on top tier(s) until after stacked.
HTH
Hi there, welcome to CC. It is not necessary to use boards or dowel for that small of cakes, I never have and you can see my cakes in my photos. If they were much larger cakes then I would not take a chance but I did not dowel or use a board for any of my doll cakes or any two layer cakes. Best of luck to you!
I strongly suggest that you do use a board under each cake layer and put dowels under the 6". If you have added weight to the bare cake by torting and/or filling, if you need to move the cake from the place where you've assembled it, if you have applied decorations and borders to the base of the 6", if you use an icing that will show cracks if the cakes shift, if you want ease of cutting, if the cake may sit in a warm/humid environment for awhile before serving, if the cake will be transferred between refrigerator and counter once stacked--all of these provide reasons for boards.
If you place each tier on a board (3/16th inch foamcore works best for me, but corrugated cardboard covered in saran wrap, etc. is another option and two of those together is even better), you can move them around so easily whenever you need to pick one up. You can put a circle of parchment on top of the 9" and that is a great target for centering the 6". You could probably get away with 4 wooden dowels (6 is best) or 2-3 plastic dowels. If the cake isn't travelling, no central dowel is needed. If it will have a car ride--even on a lap--a central dowel through both layers would be great.
Rae
Another tip--if you sprinkle sugar or coconut between the two tiers, it will keep the icing from the bottom cake from sticking to the top cake when you cut and serve it.
Also, you asked about decorating the cakes before or after you stack them. I ice the cakes before I stack them, but decorate them after they are stacked, otherwise, I might smush something while trying to stack them.
Heres a link to cake doweling - helped me alot.
http://www.cakecentral.com/article23-Teired-Stacked-Cake-Construction.html
Hi and Welcome!
Everything BlakesCakes said-ditto for me! I don't know about you guys but my 6in cakes seem too heavy even without torting and filling to stack without any kind of board or dowel support. IMHO, I don't want to take all the time to bake, make icing, decorate only to have my creation sink, slide or otherwise take a trip to some dark, ugly cake place of no return! 
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