I am making a 8,12 in cakes. I will be using Wilton spiked pillars. I made a gumpaste topper using a half styrofoam ball. It is pretty heavy now. I am worried about it tipping over. What do you think? I know I will have to use dowells in the 8 in.
Once you set it up, assuming everything is all level, you should be fine. If the cake isn't moving, and you have dowels in the 8" to help support the styrofoam ball topper, I dont' see any problems.
If you're talking about during transport, I'd transport the two tiers separately (since they are pillared) and assemble when you get there. (Then paragraph 1 applies. )