You can only order on plate of food and, one drink per sim at a time. The small sim icons show you if you have ordered a food and a drink for them. Choosing multiple items in a category (food or drink) ends the same as if you ONLY chose the last item ordered.
That's because any Appetizer, Entree OR Desert will fill the hunger need so, you only need one. However if you want more, you can order again after your sim eats and drinks the first things ordered.
That is by design, it's supposed to be that way.
