Obviously, the old lady and the doctor are the main characters. The old lady might be someone in need of medical help or some sort of assistance that the doctor is supposed to provide. Their interactions and what happens between them would form the core of the story.
The main characters are clearly the old lady and the doctor. They are the ones around whom the story is likely to revolve.
The two main characters are the old lady and the doctor. The old lady could be presented as a person with certain qualities, like being kind or perhaps a bit stubborn. The doctor, on the other hand, might be portrayed as knowledgeable but perhaps having some flaws in his character or approach. Their relationship and the events that unfold because of it are what make up the story.