If you’ve never given much thought to what a mandolin and piano would sound like together, sans any other instruments, then you’re not the only one—it’s an unusual mix. But if you wanted to experiment with that concept, then these two guys would be the ones you’d choose to try it out. Chris Thile, with both Nickel Creek and the Punch Brothers, has always been interested in pushing the possibilities of his instrument. (His last duets album was with bassist Edgar Meyer.) Brad Mehldau, one of the most inventive jazz pianists of the past couple of decades, has made a career out of pulling surprises out of his hat, covering tunes by Oasis and Radiohead as well as the more standard fare of Cole Porter and Miles Davis. This two-disc, one-hour studio set, which follows a handful of live dates, sees them melding their ideas seamlessly and single-mindedly on both original tunes and choice covers, like Dylan’s “Don’t Think Twice, It’s All Right” and Elliott Smith’s“Independence Day.” Thile is the better singer of the two, but the heart of the session is in the instrumental interplay. Mehldau, in particular, is a musician for whom category is the enemy; he’s always bolting away from expectations. Thile is a dazzling player, who, at times, seems content to let his collaborator make the big decisions but then, every so often—starting with the Jeff Buckley-esque lead track, “The Old Shade Tree”—takes matters into his own hands, guiding the music to places that mere mortals simply never would imagine it could go.