Let's take a moment to praise that oddest of instruments: the doubleneck guitar. Popularized by Jimmy Page, it's essentially two guitars in one--in his case, a 12-string (used to play the thick chordal strummy parts of "Stairway" and other Zeppelin songs) and a 6-string (used on other parts, particularly solos). I've recently stumbled across some used Danelectro models, including an unusual combination of regular and baritone. (A baritone is tuned a fifth lower than a standard guitar and is often used on old country recordings...think Johnny Cash walking along the train tracks and you'll hear it in your head.)
But in my mind, Junior Brown is the undisputed master of the two-in-one guitar. He's a monster rock/country/rockabilly guitarist from Texas who got sick of switching back and forth between regular and steel guitars. One night, in a dream, he saw a combination of both, so he set out to build it himself. The result: see for yourself.