My adult daughter and I often share at table service restaurants where you order from a menu. Sometimes they will plate the meal in the kitchen and other times the server will bring an extra plate and we will divide it ourselves. As far as I know none of the restaurants in DW have a split plate fee.
You cannot share and everyone will be charged at buffets (e.g. Crystal Palace, Boma), all you care to eat family style meals (e.g. 'Ohana, Garden Grill), the 3 dinner shows (Hoop De Doo, Spirit of Aloha Luau, Mickey's Backyard Barbecue), fixed price venues (e.g. CRT, Akershus), Fantasmic and Candlelight Processional Dining Packages, special events (e.g. Dessert parties, Dine with an Imagineer) and the Family Platter at Whispering Canyon Cafe.