O'hana and Crystal Palace are buffet/family style, so no sharing is allowed. One TS credit required for each person at the table, no matter how much they eat or don't eat.
Otherwise, sharing is allowed on the DDP at most non-buiffet, aycte, or family style restaurants. A small handfull of restaurants, most notably Via Napoli, do not allow sharing when on the DDP, however.

Not at all. You can share any entrée, and most times the chef will split it for you. Just tell the server that is your intention, with the appetizer as well as your entrée.I also greatly appreciate this question coming up. My mom and I have a res at Le Cellier. I've wanted to try it for a while and really want to try the cheese soup. I was seriously thinking that she and I could each order an app, share an entree and side, then order 1-2 desserts to share. I know one of the pp's said they've split entrees at most of the signature restaurants, but does anyone think that sharing a steak might be an issue?
When DW and I are visiting and are a TS, often the portions are much larger than we really need and by the end of our visit, even when trying to be "good" we bring back more pounds then when we arrived. Does Disney frown on placing one order and then requesting a plate so an entrée can be shared? I can understand that they may charge for the extra plate, and that would be OK, but just wondered if any of you ever do this.
Not at all. You can share any entrée, and most times the chef will split it for you. Just tell the server that is your intention, with the appetizer as well as your entrée.