You obviously cannot share a plate at any buffet meal at any Disney World restaurant. At the others, the only one that will charge is Shula's. Every other places is fine with split meals. But I warn you - most of the time, you will get the entree split (if it's a steak, half for you, half for your mother) but the sides are usually in full. I haven't done this with anything except fries but I know each of us got a full serving of fries. And then there was the time I said three of us wanted to share the cheese soup at Le Cellier. Did it come with one bowl and three spoons? Nope - each of us had our own half filled bowl and we were charged for one serving.
Disney will not let you starve.