Two pop titans are hitting the road, playing their own songs and swapping a few along the way, much like Sting and Paul Simon did a couple of years back. It’s fun to imagine the juxtapositions (“Red Rain”/”Shadows in the Rain”?), what an equal trade might be (“Solsbury Hill” for “Fields of Gold”?) and what might make for an absurd duet (“Russians"? "Big Time"?).