Hi again Sara! You're right -- the Disney Dining Plan really does give you a lot of food for the money!
At buffets, fixed-price, or all-you-can-eat meals, sharing of meals is not permitted, whether you are on the Dining Plan or paying out-of-pocket. At table-service restaurants, my understanding is that the official Disney policy prohibits sharing meals on the Dining Plan. However, many guests report that they have been allowed to share a meal and pay with only one table-service credit.
If you and your travel companion don't feel hungry enough to order two meals, I recommend that you tell your server and ask if you could share one meal instead. Disney wants its guests to be happy, and the cast members try very hard to be accommodating.
If you have more questions, please don't hesitate to ask! Wishing you a magical trip!