I was thinking of making a cake where one of the tiers is 4-2" layers high. What's the best method to make sure this is stable? I was thinking of using two cake rounds (one for each 2 layers of cake), and then using a dowel to steady it.
Stack those two tiers as you would if they were different sizes. Board and dowels under every 4 inches - or 2 layers - of cake.
When I make a double tier like this, I cut the board between the tiers a little smaller than the diameter of the cakes so it can be iced over easily.