I would say the issue is more one of how big the meals are and not whether or not they will let you share, i'm sure none would say no. DH and I shared on our last trip at the following restaurants:
San Angel Inn: We shared an appetizer, main course and dessert. They were very good and brought out two plates for us and it was enough for the both of us and quite tasty!
Wolfgang Puck Cafe: Again, we shared an appetizer, main course and dessert. They weren't overjoyed at us sharing but they did allow it!
I would say one thing we found helpful when sharing was to tell the server up front and let them know would make it up to them at the end. We were on the dining plan and tips were included so obviously they got less if only one TS credit was used so we'd leave an extra $5 or $10 on top of the 18% they'd get from what we ordered.
I also think CS places are easier to share at and some of the portions there are very big. Places like Cosmic Ray's, Sunshine Seasons, Cantina di San Angel, Tangierine Cafe, and most of the breakfast platters at the resorts were big enough to share as well!
Restaurants where we could not have shared (and would not as the food was too good!) as the portions weren't big enough were places like Le Cellier and Kona Cafe. Although the servings were big they wouldn't have been if we'd have had to share them.
Hope this helps!
Eloise