I have a wedding cake to do which will have a base cake that is 18" - I purchased the half round from GSA (FAST shipping, btw).

My question is for those that have used the half rounds - when you put the cake together, do you put the halves on the 2nd layer opposite the way the bottom layer halves are?? - will that make the cake more stable?

Yes You'll have to put them in opposite directions or you'll have a chance of a crater in the center. Might help to do a crumb coating after assembling them to get them more stable. Remember to ice the edges when you assemble them so they stay together. Use a heavy base and you'll do fine. A cake with this size base will be heavy so I never stack until on site.