No. While two 12" pizzas have more pizza than one 18" pizza, it's possible for the two 12" pizzas to be a better value.

If the total price of the two 12" pizzas is $7.20, for example, they are a better value than the 18" pizza once the price of the 18" pizza is greater than $8.10. More generally, the two 12" pizzas are a better value if the 18" pizza's price is higher than 112.5% of the two 12" pizzas' total price.

But that's almost never the case.

How do you know, unless you do the calculation?

Don't know about you, but I have never seen any place that charges more for one large pizza than for two medium pizzas.

For example the place by me charges 11 for a 12 inch pizza and 25 for a 24 inch. That's 4x the pizza for a little over twice the price!