Is two fourths of CSN&Y only half as good as the full team? Maybe that's the lesson from David Crosby & Graham Nash's first disc together in 28 years in Crosby & Nash. As expected, the stellar harmonies and feeling of folksy plangency are as impressive as they are impregnable. If only the material were more compelling. In places (as on the soaring ''How Does It Shine''), everything clicks and you're left with a feeling of warm, fuzzy deja vu. Sadly, such moments are in short supply on this overstuffed double CD.