funny you should ask ... I learned the hard way not to do that when I made my son's birthday cake this past January.... it was my first stacked cake. I made a 6 and an 8"....... I made both cakes, soaked them with syrup and torted them. I only put 4 dowels to hold up the cake .. according to my sister I should have used at least 6 and should not have soaked the bottom layer. This was my first experience with stacked cakes. This past Monday my wilton instructor said that whenever you stack cakes you should put the support dowels to hold up the board and one long one through the center of both cakes. Mine started sliding ... they were covered in fondant and the fondant craked in the bottom layer 
....fortunately all the kids were arround the cake when someone noticed ....it was assumed that one of the kids had stuck their finger and caused the rupture .... everyone laughed about it
My 50s cake that is in my photos was not doweled. I used a 10" round plate under the hexagon layer and put it right on top of the 16" square. I had no problems with sliding. It traveled 15 miles from home. Guess it's whatever you are comfortable doing.
I made a 2 tier cake in my Wilton level 3 class but I haven't ever made a stacked cake. What is the process for stacking cakes? For example - if I had a 6" and an 8"? i've done 6 inch and 8 inch a lot without doweling them. i usually do a 2-layer 6 inch and a 3" deep 8 inch and they survive. just bc frosting, haven't had to travel much with them
if it doesn't frame a kid at the party .. it worked for me 
... lololol
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