Excellent response (Mauley)!
Tertius was Paul‘s secretary who put on paper what Paul was saying. It was common in ancient time that people used to have secretary to write doing what they wanted to say. Paul is the author of the letter (Romans 1), however Tertius was putting in writing what he was saying. If you read Jeremiah 36, you see the same thing. Baruch was Jeremiah secretery to whom Jeremiah dictated the word of God.