Yes, at most TS restaurants you can share an entree and dessert and pay OOP for anything that is not covered by the dining plan.
You cannot share and will each have to use a credit at buffets, all you care to eat family style restaurants, the pre-paid dinner shows, CRT, the skillet at Whispering Canyon Cafe, and dining packages such as the one for Fantasmic or Candlelight Processional.
The only restaurant that has consistently not allowed 2 people to share a dining credit is Via Napoli.
Everyone does not have to use a dining credit. By sharing credits people are able to save credits for additional meals or for 2 credit meals.