UPDATE: After reading Kakeladi's post I thought, that's a heck of a lot more than what I estimated so I went back to my notes. Sure enough, she's right. What I quoted in my first post was for filling the 10 inch round only. I decided to do a bit of math and the numbers add up close to Kakeladi's recommendation so FOR OTHER FOLKS WHO MAY SEARCH THIS QUESTION, you'll need way more BC. here's what I got (and PM'ed OP):
The last time I measured my SMBC I got 5.5 oz per cup. Depending on how much you use for filling, you could use 1.5 cups to 2 cups per layer of filling for the 10 inch-- meaning between 4.5 to 6 cups to fill the 10 inch round = 24.75 oz to 33 oz for filling the 10 inch round.
For a 12 inch round I would estimate an additional 3/4 cup per layer i.e. 6.75 cups to 8.25 cups = 35.75 oz to 45.4 oz
Total fillings: 60.5 oz to 78.4 oz
The average 6 egg white batch of SMBC (1:2:3) yields about 34.5 oz BC
To cover and decorate the 10 inch and 12 inch cakes, I would guestimate a quadruple batch i.e 138 oz
Total for both cakes: 198.5 oz to 216.4 oz i.e. between 5.75 to 6.3 BATCHES of the basic recipe.
Hope that helps.