The Milk Carton Kids are really just two Los Angeles-based singer-songwriters — Joey Ryan and Kenneth Pattengale — who figured out that they sounded good playing together. But to the rest of the Americana world, the two are magic in a bottle, blending the fraught folk weariness and preternatural guitar picking of Gillian Welch and David Rawlings with the fragile pop harmonies of Simon & Garfunkel.