The email response I received from Disney stated that guests on the dining plan are not required to use their dining credits for the meal, and may pay for the meal at the time of dining. They also said, "If one Guest wishes to use a dining credit and the other Guest in the party wishes to order food and beverage and pay for it instead of using their dining credit, that is permitted. However, ordering one meal and sharing is not permitted with the dining plan."
But that's just an email I received, it's not in a brochure or anywhere else I've seen in writing. You can always try emailing Guest Services to see what they tell you. If they are just cutting and pasting a generic answer, you'll get the same answer I did, verbatim.
DH and I wanted to share meals for exactly the same reason you stated: portion control, not wanting to waste food, we usually share appetizers and desserts anyway,
if we even order them. However, after doing the math -- ordering a second beverage and entree at lunch, and ordering a second beverage and just a second appetizer or dessert at dinner, it looked like it would cost us an additional $20-$25 per day,
in addition to the cost of the plan, and we might end up with unused credits at the end of our trip. Not financially sound. It makes more sense for us to just use our full credits, eat whatever we want of the meal, and leave the rest on the table, even if it is a waste.

We figure if the dessert we order is reasonable to carry-out, we might ask for it to go, so we can actually enjoy it later instead of taking 2 bites and leaving the rest. Other than that, it's not really practical to try to carry leftovers around the parks, have no way to reheat them in our room, and I'm sure not taking them home as souvenirs
