I am planning to make a two tier round cake (6" and 10"). I read in a cake decorating book that one should put a thin cake board under each layer of the same size as the cake...and then a thick board beneath all. I totally get the need to put a board the same size as the small layer underneath it...but can't I just place the 10" layer directly onto the thicker, covered cake board? I am planning to decorate the cakes with fondant (my first attempt!). Thanks in advance!
Yes, you put the bottom tier directly on the cake drum.
I did the same for my last two tier cake. A plastic thin cake board under the top layer, and then I placed the bottom layer directly onto the thick cake board.
but can't I just place the 10" layer directly onto the thicker, covered cake board?
Yes, but it depends on what your board is covered with. If not foodsafe, you'll want to use the 10" circle so the cake doesn't actually come into contact with a non-foodsafe covering such as clear contact paper, wrapping paper, or certain non-foodsafe colored foils.
Also, when you apply BC (before your fondant), if you're one who tends to make a mess and would rather not have to wipe off the excess from your board or drum, you could transfer the cake to the board after it's iced but you'll need the cardboard to help prevent your cake from cracking.
Other reasons to use the extra board would be to protect your plateaus or plates from knife scratches if not using disposable drums or boards, etc.