David Olney and Steve Dawson, who share a stage this weekend at Off Broadway, have nothing and everything in common. Olney is a Southerner, and Dawson a Yankee, and their audiences rarely overlap, as the former has been amassing one of the strongest catalogs of any post-Dylan songwriter, while the latter is best known for his work with the alt-country (if you must have a label) band Dolly Varden. But their songwriting shares an intensely visual quality, flashing with wisdom in scenes that each man frames like an artful director. Plus, both Olney and Dawson owe a great debt to blues, country and soul music; they understand the importance of rhythmic drive, even when just performing with acoustic guitars. Together they make for what could be the songwriter double bill of the year.