If it's just the 9" round on top of the 9"x13" then no, not if you don't want to. If you're using the wonder mold, yes. There's a good tutorial on how to dowel a cake in the articals section of the site... it's the one on the construction of a stacked tiered cake.
I would definitely use dowels on this. I sometimes go for overkill, but IMHO a 9" cake is gonna be pretty heavy on top of that sheet cake. It would probably end up falling right through. As far as how many dowels, I think a lot of people seem to use 1 dowel per inch of cake, so in this case 9. What you would do is stick the dowel into your iced sheet cake, marking where the top of the icing hits the dowel. Cut that dowel to that length, make 8 more the same size. Mark the lower cake, using a cake board or even the pan that's the same size as the 9" you're sitting on top. (just press the board lightly into the icing to make a slight indentation). Put your dowels about 1 inch inside where the edges of the 9" cake would be, making a circle. Try to keep them as evenly spaced as possible. Cut a piece of wax paper the same size as your top tier and place that between tiers. I've never tried but hear it keeps the icing from sticking to the upper tier. I would then cut a dowel the height of the stacked cakes (how high they are when assembled), sharpen one end, and stick it through the center of the upper cake, through it's cake board, through the lower cake and into that cake board. Like I said, it might be overkill, but I like to be safe.
HTH
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