If it's for the character buffet breakfast then I'd say no. I have to eat gluten free which limits the number of buffet items I can eat drastically and there's no discount for that. And if they allowed someone to pay less because they ate less you'd have a lot of people saying they 'eat less'.
For character meals, you're paying more for the experience than for the food.
If you're talking about when you can order off the menu (so lunch/dinner) then you can share/split menu items, but still not the buffet.