Multi-neck guitar
A multi-neck guitar is a guitar that has multiple fingerboard necks. They exist in both electric and acoustic versions.
Multi-neck Guitar Media
Mike Rutherford of Genesis, circa 1980, playing his custom Shergold guitar in its twelve-string/bass double-necked configuration
Chris Squire of Yes (2003) playing Wal triple-neck bass consists of 3 double-course guitar, fretted and fretless 4-string basses.
Dave Mustaine of Megadeth playing a Dean twin-neck. Note the machine heads for the 12-string secondary strings on the edge of the body.
Jeff Martin of The Tea Party playing a Gibson EDS-1275 in Canada