AI'm doing my first tiered cake (double layer 12" with double layer of 8" on top) and I'm going to put the top tier on a cardboard round for support. (with dowels also) Should I also use a cardboard round under the bottom 12" tier? Or attach it right to the covered Masonite decorative board? I'd like to keep it on the cardboard round while I'm decorating it, the move it to the decorative board when I assemble it- is that right? but then how do I attach the two boards to each other?

Yes, keep the 12" on it's own board.  You can use royal icing, glue dots [my personal fav], double sided tape, etc. to stick it to the decorated board.

AThanks! I guess my real concern is about. transporting the cake. The glue dots are strong enough to keep the two boards together for a 30 min car ride?