When there’s no brain-massaging kick beat or snappy guitar delay to mark the time, The Orb and David Gilmour’s two-track collaboration Metallic Spheres gracefully drifts to its peak. This stands to reason, because both artists have a sense of timing that’s brilliantly suited to vast, lengthy formats. Between the 14- and 24-minute marks on “Metallic Side,” the ambient ravers and the Pink Floyd guitarist manage to recall the folkier moments of Ummagumma and Meddle, and even the playful expansiveness of U.F.Orb, amid backdrops that glitter and drones that warmly glance off each other. But as an Orb production, the greater parts of “Metallic Side” and “Sphere Side” feel slow and leaden, both reaching failed peaks with strangely halfhearted chants about justice and human rights. Gilmour’s electric passages sprinkle on the swoops, echoes, and other unmistakable trappings, without much of the keen melodic purpose that muscled through both halves of “Shine On You Crazy Diamond” and “Comfortably Numb.” Of course, this isn’t about grandiose solos, and Gilmour does weave his style into The Orb’s bigger picture almost perfectly during the first, minimally hooky few minutes of “Metallic.” After a while, though, it’s a bit like hiring a master painter to doodle a flock of birds into the background. Gilmour and The Orb meld enjoyably enough in their comfort zones. If only they’d focused more on pushing beyond.