It's a guitar that has two sets of fingerboard that enable the guitarist to play two guitar parts simultaneously as he/she touches/taps the strings against the frets. I'd like to differentiate the term between "double neck guitar" and "double guitar". Subjectively I find "double neck guitar" describe more of the physical property of the guitar. A lot of guitarists play double necker, but they play the neck each at the time, (one neck played, other neck's idle, or only open string/s) they don't play it simultaneously. Where else  "double guitar" functionaly serves a meaning as a guitar that sounds like two guitars simultaneously as the guitaris plays on both necks at the same time.