Who were Sue and Johnsy? How did they get acquainted with each other?

In O. Henry's short story "The Last Leaf," Sue and Johnsy are a couple of struggling artists living in a shabby apartment in Greenwich Village. At the time the story was written, the Village had a somewhat bohemian reputation and was a magnet for artists and creative types of all kinds. The narrator of the story gives us a practical reason why artists value the neighborhood—they can easily find cheap rooms with plenty of light.
Sue and Johnsy are two such artists. They originally hail from Maine and California respectively, and they first meet at a restaurant on Eighth Street one day in spring. They soon discover that they have an awful lot in common. They like the same kind of art, the same kind of food, and they share the same taste in clothes. So Sue and Johnsy decide to live and work together.