Given their nearly fifty years of friendship, and all the Farm Aid appearances, onstage jam sessions, and late-night hangs that implies, it’s weird to think that Bonnie Raitt and Willie Nelson have spent such little time in the studio together. In 1979, the two hooked up with Leon Russell for a loping cover of the jazz-blues standard “Trouble in Mind.” In 2002, Raitt harmonized with Willie on an eloquent reading of a pensive pop ballad about regret, “You Remain.” But the hands-down high point of their studio collaborations, and as beautiful a recording as either ever cut, was their 1993 duet “Getting Over You”—which became the cornerstone of one of the most important albums of Willie’s long career, Across the Borderline.(Read a transcript of this…