At TS restaurants you can share when you are at a restaurant where you order from a menu.
You cannot share and you will each be charged at buffets, all you care to eat family style restaurants (e.g. 'Ohana, Garden Grill), the 3 dinner shows (Hoop De Doo, Spirit of Aloha luau, Mickey's Backyard Barbecue), fixed price meals (e.g. Akershus, CRT), dining packages (e.g. Fantasmic! or Candlelight Processional Dining Package), dessert parties, special meals (e.g. Dine with an Imagineer or Animal Specialist) and the Skillet at Whispering Canyon Cafe.