Tone is defined as the attitude that a writer has toward a subject or an audience. The author can have a sad, happy, comic, etc. tone towards a subject and convey this through his writing. The most common way in which a writer conveys this is through his choice of words.
It is true that two authors can develop the same tone, because more than one author can share the same attitude toward subjects and audiences. In fact, this is one of the main ways in which we classify literature. Authors who have the same attitude towards a subject are often considered to belong to the same movement (eg. the attitude of the Romantics towards nature).