Leave the cake board under the 2nd tier. But, you will need dowels or bubble straws inserted into your first tier to support the 2nd. Make sure all the the supports are exactly the same length or you will have a leaning tower of Pisa.http://www.cakeswebake.com/forum/topics/about-stacking
You do need to keep the board under it... If you want to invest in a great video... Sharon Zambito's Successful Stacking is a great video... all of her videos are fantastic!
I leave the board under the smaller tier too. The rods go into the lower tier and the board ends up resting on them.
This helps a lot in supporting the whole stacking structure.
Always leave the board and make sure that you use plenty of support, I like the PolyDowels as they are so easy to use http://www.ahcakedesign.com/cakesupports.html. If you are going to stack 3 or taller use a long dowel that is sharpened to go all the way down to your base to keep the tiers from sliding.
ok-when i put the cake on the board to cover in fondant or frosting do i cover the board in foil or something else o r do i leave it? Also with the long dowel going all the way thru the layers doesnt bring cardboard, foil etc... with it? Sorry for so many questions. I have a b day cake coming up and it's gonna be big so I just want some to make sure i am doing it right. Thanks