The ‘tapping’ method is an advanced guitar technique where both hands are placed on the fretboard, typically reserved for solos. Who was the first guitarist to use this method - live or otherwise?
First guitarist was possibly Jimmie Webster, who made recordings in the 1950s using a two-handed tapping method he described in 'Touch Method for Electric and Amplified Spanish Guitar', published in 1952. George Lynch, from the bands Dokken and Lynch Mob, has claimed that George Van Eps used tapping in the 1950s as well.
As far as Rock goes, the earliest purveyor mentioned is usually Canned Heat guitarist Harvey Mandel, whom none other than Ritchie Blackmore attests to have seen perform this method going back to 1968. Randy Resnick (who was mentioned in the Eddie Van Halen biography) of the band Pure Food and Drug Act used two-handed tapping techniques extensively in his performances and recordings between 1969 and 1974. Resnick also recorded using the two-handed tapping technique in 1974 on the John Mayall & the Bluesbreakers album "Latest Edition". You might note that these were all California musicians, and would have been bands Eddie VH was going to check out on weekends.
However, just to prove this phenomenon didn't stay in California, take a listen to Steve Hacket (Genesis)'s work on "Dancing with the Moonlit Knight", from 1973, and "The Return of the Giant Hogweed", from 1971. So, either the wheel was being invented on 2 continents simultaneously, or somewhere during tours one band heard the other and began employing it as well.