David Bowie made a holiday perennial with Bing Crosby and a crime against humanity with Mick Jagger, proving that duets have no logic. Until voices intertwine, there's no certainty, only suspicion. Mark Knopfler and Emmylou Harris seem oddly matched, but the tension between their vocal personas—she's ever hopeful, he's relentlessly glum is the highlight of All the Roadrunning. On the up-tempo ''This Is Us,'' one of 10 Knopfler originals, they trade lines and harmonize like a spicy married couple, which makes it all the more disappointing when Knopfler locks himself in the toolshed with his six-string mistress. Too often, the Dire Straitsman and his guitar flirt at length while Harris' levels are turned down until she's just providing distant backup.