I'm making a 2 tier BC cake 8 in. and 6 in. Do I put one dowel down the center so it goes through both tiers or do I put several shorter ones in only the 6 in. cake or do I do both?
Four dowels in the bottom tier, next tier on a cardboard or foamcore circle. Then if you are going to transport the cake anywhere, use a center dowel. I only use one..hammer it all the way through the bottom board.
Here's the wilton step-by-step if you find pictures helpful: http://www.wilton.com/cakes/tiered-cakes/stacked-tiered-cake-construction.cfm
I never use a center dowel at all, and for a two-tier cake it's really unnecessary.
If you're transporting it just keep it in the fridge until it's ready to go. As long as you move it without banging it around it should be fine.
The dowels go in the bottom tier to hold up the top tier. Sticking all the dowels just in the top tier won't do anything but fill that tier with dowels.
I agree with Texas_Rose.