I've made a few of these types of cakes.....I put a cake circle under the 1/2 ball pan for stability. Then I use about 5 dowels for support in the bottom cake.....normally I just use lolly-pop sticks since the top cake isn't that heavy.
Are you planning on transporting this cake past your front porch ? If so you can either put it together when you get to the venue, (remember there is always touch up work when you assemble a cake and not everyone feels comfortable/experienced enough to do this with spectators) or you can dowel the bottom for support and then sharpen a dowel that will go all the way through the ball cake and the base cake and touches the bottom cake board.
Jibbies
I would definatly use dowels!you never know!
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