Amy Ray and Emily Saliers are the Indigo Girls. Rolling Stone describes them as the “ideal duet partners. Their voices soar and swoop as one, alternately raucous and soothing. When they sing together, they radiate a sense of shared purpose that adds muscle to their lanky, deeply felt folk-tinged pop songs.” Together they write, arrange, record and perform music that has become a vital part of the lives of their legion of devoted fans around the world. Charleston Music Hall hosts the Indigo Girls September 8.