DBF and I shared the spaghetti and "meatballs" for lunch when we went in December. They brought out two plates and we split it ourselves. There was no fee, although I was teased for sharing the peanut butter and jelly milkshake with him!
DBF and I almost always share a meal when we eat at Disney. That way we can split an appetizer, entree, and dessert without wasting food, saving money, and getting to try a little bit of everything. Sometimes you'll get a waiter with an "attitude" who is obviously unhappy you are splitting but other then that, we've never had a single problem!