Customization
There are many variations of how multiple-necked guitars can be customized, such as the number of strings on a neck, frets or no frets, the tuning used on each neck, etc. Recently, instruments with two or three necks that combine 6, 7, and 8 string guitars have become popular.
One of the more common combinations is where one neck of a double-necked guitar is set up as for a 6 string guitar and the other neck is configured as a 4 string bass guitar. Guitarist Pat Smear of the Foo Fighters utilizes a double-necked guitar during live performances (bass guitar top neck, six-string electric guitar bottom neck) in order to perform Krist Novoselic's bass part in the song "I Should Have Known," from the album Wasting Light, in addition to his own duties. Rickenbacker International Corporation and Gibson Guitar Corporation in the USA have both manufactured production models of these configurations in the past.
A less common configuration has a 12 string guitar neck combined with a 4 string bass guitar neck: Geddy Lee of Rush is well known for using the 4/12-string Rickenbacker 4080/12 production model live in the 1970s.
In the 1970s and 1980s Mike Rutherford of Genesis was known for playing a custom-made Shergold Modulator twin-neck guitar-bass unit in live shows, as he frequently changed between lead guitar, 12-string guitar and bass guitar, depending on the arrangement of the song. The unique design concept of Rutherford's Shergold guitar set is that it consists of several interchangeable modular elements, each of which could be separated and recombined, and which attached to the other units through a system of dowels and thumbscrews, and an electrical connector. The complete original set consisted of a "top section" 6-string guitar, two "top-section" 12-string guitars (each in a different tuning), and a "bottom-section" 4-string bass. Each section could be separated and re-attached to create a variety of twin-neck combinations, with either the 6-string or one of the two 12-strings on the top, and the 4-string bass on the bottom, and there was a matching lower-body section which could be attached to the 6-string and 12-string main units, which created a single complete guitar when these were not attached to the bass. (As a tongue-in-cheek reference to this, the puppet version of Rutherford in the video for Land of Confusion plays a four-necked guitar.)
Read more about this topic: Multi-neck Guitar