Which is better deal 2 8 inches pizza for 10.99 or one 12 inches pizza for 9.99?

1 12 inch Pizza for 9.99 is the better deal.

Whether or not we consider the total volume of pizza purchased or just the surface area of pizza (which reflects how much topping there will be) the proportion between the 12 inch and the two 8 inch pizzas (combined) always results in 9:8.

So you are getting slightly more pizza with the 12 inch pizza, and it is cheaper, so it is the better deal.

Volume = pi * r2 * h. As r2 for the 12 inch pizza = 62 = 36, this is more than 2 * 42 = 32 by the proportion 36:32, which simplifies to 9:8.