Double neck guitar may be:

• Gibson Double Neck (prototype) 70001 (in 1976 list),  or
• Gibson Double 12 (1958-1962, custom-order twin-necked hollowbody guitar, predecessor of EDS-1275 )

References

• Lowell Cauffiel. " Stephen Stills ". Guitar Player (January 1976). " Gibson Double Neck (prototype) 70001 "
• Gibson Custom  EDS-1275 Double Neck . Gibson Guitar Corporation . " Body: From 1958 to 1962, the Double 12 double necks sported a hollowbody design crafted with carved spruce tops and maple back and sides. The body also consisted of a Florentine double cutaway design which provided easy access to the entire fret range of both necks. In 1962, however, the Double 12 was reintroduced as the EDS-1275 model and featured a solid mahogany body that closely resembled Gibson’s new SG models, along with three-piece maple necks with 20-fret rosewood fingerboards. ... "