How much batter is needed for two 8 inch rounds? I thought that I saw a chart somewhere on here before, but I can't find it now.
Here are all four Wilton cake preparation and servings charts:
http://www.wilton.com/cake/cakeprep/baking/times/index.cfm
These charts give batter requirements by pan size (2 or 3" depth) baking temp. and times. Also give wedding and party size servings by pan size.
FYI, sheet cakes are listed under party cakes charts.
HTH
Here this might help:
http://forum.cakecentral.com/cake-decorating-ftopict-22705-.html
It's a forum on this same topic.....I use this reference page every time I make a cake.
Here is something that may help you; after you find the batter chart.
On the outsiide of your pan with a permanent marker write 31/2 cups or whatever that particular pan takes; then you will always have the reference in front of you. It stays on through dishwashing and since it's on the outside of the pan it never comes in contact with the cake 
Jibbies
If you use the cake extender with one box mix, you can fill two 8" rounds with one recipe.
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