Use dowels cut to the height of you cake and then another long dowel to go down the center so they will stay. If you look in "articles" here on CC, there is a great tutorial on how to do it. It shows you step-by-step how to do it with pictures. HTH 
I always use dowels .....I have been using the wooden ones but I think I am gonna switch to the plastic kind..... Anyways I stick the dowel down in the cake to get the height of the cake and then I cut it off where the icing ends then I make several more of the same size, then I just make a mark on my bottom cake where it is going to be sitting and then I make sure to put a dowel about inch to inch and half where the cake will be sitting.
HTH......I hope I didn't confuse you....I had to stop and answer the phone while typing this.
For my first stacked I put my first cake on a board & then dowels underneath. Just take the the top layer's board & "draw" in the icing so you have a template of where to put the top cake. Then I just read don't put the dowels all the way in the cake, leave them sticking out just a little & then put the cake on top. THis is where I had problems (putting the top layer on w/out messing up the bottom layer). Hope this is the kind of stacking you are doing.
Oh, I don't know what people use to cut a dowel, but I would recommend some type of a saw--at least a small hacksaw.
If I understand correctly leily said on another post to stack, carve, (score along the side to give a guideline), disassemble, dam & fill, restack (using boards if necessary. This way your dam doesn't break & you filling doesn't ooze out.
Won't that get messy or ruin the decorations?
It would be messier trying to ice a stacked cake. If there are certain decorations that will be affected during the stacking process, do those after. But ice the cake first.
Since they are just 2 rounds ice first then stack. Another tip I saw that I'm going to try is leave a little of the dowels sticking up above the cake & they will go the rest of the way down when you put the cake on top of them. This way you won't have to worry about your fingers messing up the bottom layer
dandelion56602, I have posted several times about leaving the dowels up slightly. It works GREAT! Be sure your dowels are in perfectly straight and they are all at the same height before you stack. I put a cake board on them and use a level to be sure they are all where they should be. Good luck - let me know how it works for you!
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