Two handed tapping is usually on one string and you will be mixing two melodies together, one by tapping a treble pattern on the right hand, and one by using a kind of platform on your left hand eg:


19p12-t-15p12-t-19p12-t-15p12...

(I don't know if that actually makes sense, it's just to show you the pattern.) (What you'd have is the positions of each different tap or pull, corresponding to a major or minor chord so you're using repeated sequences showing the chord...)

the index finger of your left hand remains on 12th fret, and you use the pick on the 19th at first, tapping to make the original sound. When you pull, it goes to 12th, and when the note's ringing on 12th, you use your 4th finger to tap 15th and then pull back to 12th.

So essentially you're using both hands. You make the actual melody by moving the chord patterns whilst tapping, using more or less fingers (that example was a very simple one,) and changing the base note.

An awesome example is Joe Satriani's "Surfing with the Alien" at about 2 mins after the interval he starts some insane tapping... listen to that and you'll understand.

Take it easy. Any corrections on what I've just said would be welcome...