Ok, I plan on making a "two-tiered" cake for my mom's birthday this weekend. I plan on doing a 6" on top of a 10", haven't decided if I'm going to do single or double layers yet. Advice on this would be great. I think I understand the dowel procedure, but do I need the circle in between the two tiers? We didn't do this type of cake in my wilton class so I am a little bit lost.

You could probably get away with just a cardboard in between the 2 layers along with some dowel rods inserted into the cake.

Can this type of cake be done without using dowels or cardboard circles?

I guess it could be (I wouldn't) done w/o them but you would be making in harder for yourself! Getting one on top of the other, chance of messing up the icing, cutting and serving etc!

OK, final question, would I decorate the top layer before putting it on or after? I am worried about trasporting as I don't have a cake traveler tall enough.

You could do the general decorating on the cake before transport, and then when you get to your site, do the finishing touches.
It all depends on how comfortable you are, if you would feel less stressed to finish on site- then do onsite. If you trust your self to finish at home (or where ever your cake is made) and then deliver, do it that way.
Just make sure that if you decide to finish onsite to bring all the stuff that you will need. I have done a lot of cakes where I get the basic icing done and then add any finishing touches to the cake onsite.
Hope this helps!

The dowels are really easy. You just cut them the size that you need. You can also use straws and lollipop sticks. It's very easy, Just cut them the same size. I placed the cardboard (same size as the top tier) and made an indent in the cake. I inserted the sticks in the circle and one in the middle. It worked wonderful. Thanks to the info that I received here.

I hope it's not to late for this post but here is what I find helpful to me:

You will need at least 4 plastic dowel rods and 1 wooden dowel rod). As suggested you can use straws but do not use the ones that have a flexible top. I prefer the plastic dowel rods.

You have a 10 inch and 6 inch cake. The 10 inch cake should sit on a heavy duty board at least 2 inches wider than the cake so in your case you will need a 12 inch heavy duty board. The 6 inch cake sits on a 6 inch board, the exact same size as the cake.
Get a piece of parchment circle paper the size of your top cake (In my example I would get a 6 inch parchment circle). Center this parchment circle over your cake (the 10 inch cake) then mark your bottom cake (the 10 inch cake where the 6 inch cake will sit, mark about 4 places or you can lightly circle the parchment circle). With this parchment circle in place mark the 4 spots where you want your plastic dowel rods to go. To do this just lightly press 4 indentures into the cake.
Now remove the parchment circle and get your plastic rods and slowly press it into the indenture, making sure the rod is in straight, then mark with a edible pin where you will cut off the excess. (note: the plastic dowel rod should be the same height or a little bit smaller than the 10 inch cake). Use this rod that you have just cut to cut the other 3 rods. After you have cut the other rods place all 4 of them in the cake straight and evenly. Then for more support, I normally fill the inside of the plastic rods with frosting. Now you are ready to place the top cake (in this case it is the 6 inch cake) on top of the 10 inch cake. If you are using butter cream frosting leave the 6 inch parchment circle on the cake then place the 6 inch cake on top of the parchment circle. If you are using Fondant you can remove the 6 inch parchment circle before placing the 6 inch cake on top of the 9 inch cake. If you are using Fondant this last step is up to you on whether you leave the parchment circle down or you remove the parchment circle. Now get a long slender dowel rod and measure it from the table to the top of the cake mark this spot. Now cut this dowel rod with a pencil sharpener the height of the top cake (note: this is done for additional support especially if you will be transporting the cake already decorated and ready to go). When this is finished, place this rod in the center of the top cake (you might need a light weight hammer to assist you in driving this rod through the 6 inch and 10 inch cake) . Press the rod all the way down until it reaches the bottom board. Now cover this small hole with fondant or frosting. (If you are using fondant as your finished product be sure and place your topper or a design over the small hole. If you are using Buttercream Frosting as your final coat/product cover this hole with more frosting or a topper. I hope this helps.

Bettye