As of 2009, are Hall and Oates the best selling rock duo?

Apparently, yes -- The Philadelphia-originated duo is one of the most popular and enduring collaborations of the rock era – according to Billboard magazine they are the best selling duo of all time. They are eligible for the Rock and Roll Hall of Fame.
sing365.com/music/lyric.nsf/Hall-Oates-Biography/4336892B891A4BB148256A92000F8A5C no longer exists

Hall & Oates are the best-selling musical duo of all-time with a staggering number of monster hits, but the rock establishment has never taken them quite as seriously as the huge fan base. "It's probably because they had their own opinions about what constituted exceptional music in that period of time," says Hall. "And it was based on a lot of things that I wasn't really interested in. I have slightly different roots than most of the [rock] writers." Their induction into the Hall of Fame [in 2014] clearly shows that times have changed. Here's a look back at their incredible 40-year career.

https://www.rollingstone.com/music/music-lists/hall-and-oates-road-to-the-rock-and-roll-hall-of-fame-10344/

These two guys deserve a mention:

The unlikely twosome of childhood friends from Queens, New York, Simon & Garfunkel would go on to become the most celebrated folk duo of the 1960s. Scoring a series of hit singles and albums throughout their long and illustrious career together, the pair is renowned for their signature choirboy harmonies – a staple sound of the American folk-rock revolution. Simon & Garfunkel are best known for songs such as "The Sound of Silence" and "Bridge Over Troubled Water", while The Graduate's best-selling anthem "Mrs. Robinson" earned the pair a Grammy Award for Record of the Year in 1969. Fusing their own unique blend of folk, pop and rock styles, Paul Simon and Art Garfunkel have long delighted fans worldwide while casting a special impression on the history of popular music.

http://www.myspace.com/simonandgarfunkel

And this list shows The Pet Shop Boys as having made $100 million dollars in their careers without listing any other duos.

https://en.wikipedia.org/wiki/List_of_best-selling_music_artists