Quote from : All the President's Men Book

The August 1 story had carried their joint byline; the day afterward, Woodward asked Sussman if Bernstein's name could appear with his on the follow-up story - though Bernstein was still in Miami and had not worked on it. From the on, any Watergate story would carry both names. Their colleagues melded the two into one and gleefully named their byline Woodstein. -- Carl Bernstein, Bob Woodward