The story revolves around the conflict between a man and a woman, who are faced with the decision of what to do about a pregnancy. They view the situation differently, and they communicate differently. Hemingway is demonstrating the "two" sides to every situation through the two individuals. He is also demonstrating the difficulties in a romantic relationship, which is made up of two people.
What the two characters say to each other is also double-sided. They are both trying to express themselves, but also to please the other person. The woman says she'll do what the man wants, but she says it with a lack of confidence. She obviously wants him to defer again to her. He replies that he will not try to influence her, but he continues to do so. Everything said has a double meaning.
Finally, the characterss lives can now go down two possible paths. They can choose to keep the baby, or abort the baby. Hemingway illustrates the dual nature of any decision, showing how every choice opens up new choices. Nothing in singular.