I always use spc but just realized that will not work :) tks for any advice
i've used a wooden center rod down through all the cakes which is hot glued to 2 cake boards. there was just a thread of this a few days ago, about ron ben israel's method of doweling a cake.
I would set on a drum and then do three dowels in a triangle or four in a square on the bottom tier (i think of a double barrel just as a two tier with the same size cakes) with a cake board between the two tiers. One center dowel then staked through both and into the drum.