Greetings and salutations CC. I am preparing to make a tiered cake (8x2in and 6x2in).  I'm going to use DH yellow cake mix for each tier.  According to the Wilton batter to cake pan ratio an 8in round will need 4 cups of batter and a 6in will need 2.  I'm guessing one box mix will yield 4 1/2-5c batter. So for the entire cake I will need approximately 12 cups. The mix instructions says it will make 2 8in layers. My initial plan was to use one box for one layer of each tier. But now I'm not sure it will be enough to get a good two inch cake. Should I just ignore the box instructions and use almost one box mix per 8in pan? Will two boxes be enough for this size of cake or should I play it safe and get 3?  

There is not quite enough batter from one mix to make both the 8 and 6  - partly do to the reduced size of the mixes :(   If you use my *original* WASC recipe you will be able to get both (and maybe a couple of cupcakes).  That way you will only need 2 boxes of mix total.  

http://www.cakecentral.com/recipe/7445/the-original-wasc-cake-recipe

When I use box mixes, I have figured out that one box mix with fill two 6 inch pans. And I would use 1 box mix per 9 inch pan, and 1 1/2 boxes per 12 inch pan. Sorry I don't have 8 inch. 

I would definitely go by the Wilton chart especially if you are using professional baking pans that are typically 2-3 inches high. I used the Wilton chart to make a WASC cake for a wedding and the measurements worked out perfectly.

Please learn how much batter will be right for each pan.  It will save you the very messy work of measuring by cupful.  The biggest problem is that most people when measuring batter by cupful do not scrape all the batter out of the cup each time so the amount is not accurate.   If I can remember them right here is a list for each 2" deep professional pan:

three  6" rounds OR one 6"R & one 6" square  OR one 6"R & one 8"R- one mix;

  10"R OR 8" square OR *some* Wilton shaped pans - one mix;  

10" sq OR 12" R OR *most* Wilton shaped pans - 1 *original* WASC recipe OR 1 1/2 mixes;  

14"R OR 12"sq - 2 mixes;  

16"R OR 14" sq three mixes OR 2 batches of *original* WASC recipe.

I seldom worked w/odd numbered pans but I'm sure one could figure out how much batter would be needed based on the above list.  

Don't go by what the box says.  They are likely assuming the pans are only 1.5" high, rather than 2", and they still say the same thing even after reducing the size of the mixes.