If you it's not inconvenient to take leftovers home, you should order the largest pizza because restaurants typically price their pizzas linearly relative to diameter. A 14-inch pizza (154 sq in) is roughly twice as big as a 10-inch pizza (78 sq in), but typically won't cost twice as much, especially when you order toppings. Using the same logic, it makes no sense for two people to get separate pizzas. Assuming there are no allergies, you're better off getting the largest pizza and asking the restaurant to make it half and half if you have different preferences.
But, this only works if you're willing to take the pizza home and you enjoy eating leftovers. If you have a habit of letting food go bad, it doesn't make sense to take advantage of these unit economics; you'll just be wasting money.