I was wondering how long this technique has been used, and found this:

Guitarists using double-handed tapping are able to music that is normally reserved for pianos or keyboards, because they can accurately play eight or even nine separate notes, depending on their dexterity.

The effective note range is also increased. Double-handed tapping allows a guitarist to play two melodies, or a melody and accompaniment simultaneously, in the same way that a pianist can.

Tapping is a technique that has been used on various stringed instruments for a long time in musical history. For modern guitar playing, it is due to Emmet Chapman’s pioneering in double-handed tapping, that made the technique popular.

He discovered the technique of using two hands for tapping, by positioning them parallel to the frets. He even went as far as to invent the “Chapman Stick”. This instrument is a modified guitar, with a wide neck and up to 12 strings, designed purely with tapping in mind.
guitarplayerworld.com