When you think of sibling harmony, some artists that come to mind include The Bee Gees, First Aid Kit, The Avett Brothers, and The Jacksons. But there’s another factor that we tend to think about less — the non-verbal synergy that’s part of the equation, too. Which brings us to the instrumental duo Hermanos Gutierrez, whose intertwined guitar lines are a trademark of their sound. In the case of “Low Sun,” we get into the darker side of their music.