“I don’t know if it was ever some big idea,” says Kelly Willis with a musical laugh. “It’s just something we’ve always sort of done. Our Christmas shows were always so fun, and obviously we’ve always been part of each other’s records. But we’re also our own people. We’ve always been very careful about not losing sight of that. We didn’t want to get lost in a duo because we are so different, and we’ve each worked so hard to establish our own careers.” The thing about chemistry, though, is it can’t be denied. To hear them sing together is to understand opposite attraction, spontaneous combustion and a whole lotta life lived together. Robison and Willis—two decades in—know the way the other leans almost without talking.