My sister and I have shared burgers at WDW. Especially at Beaches 'n Cream. they have a double patty burger. We tell them that we want to share it and they cut it in half for us, place it on separate plates, and give both of us fries. Their hot dogs are also large enough for 2 people.
I'd also suggest eating at CS. Each park has CS meals in the $6 range. In MK at Columbia Harbor House, their sandwiches have a large amount of meat on them and could easily be shared.
One of the best buffet values at WDW is Trail's End buffet at Fort Wilderness. Dinner is about $18 per person. Lunch costs less and I believe breakfast is about $11. I have always found good breakfast prices at the food courts. A full breakfast is about $6 to $7.