Two great friends, two great guitarists. One’s from England, one’s from Kentucky. One’s made a name for himself playing with the likes of Jeff Tweedy and Richard Thompson. The other is known for his deep knowledge of traditional music as curator of the legendary Alan Lomax Archive. They are James Elkington and Nathan Salsburg. And they’ve recently released “Ambsace,” their second album of guitar duets. They’re playing an album release show at Constellation on Saturday, and they came by to play for us and talk about the music they’ve laid down.