Two handed tapping is a technique where the right hand hammers a note onto the fretboard followed by a pull-off while the left hand is fretting a note.
Hold the following note with the third finger of your left hand.
Then with the middle finger of your right hand hammer-on and then pull-off the note at the 12th fret of the third string as shown in the following photos. When the right hand plays a hammer-on, it is called a tap and is indicated by a T placed above the note.
Both the tap and the pull-off are played by the middle finger of the right hand. The middle finger is used so the thumb and the first finger can still hold the pick. Some players hammer the pick onto the fretboard to achieve the tap and pull-off.
This example contains the same tap as the previous one, but also includes an open string slur after the tap, and pull-off by the right hand. This is then repeated with the right hand tapping the note on the 11th fret instead of the 12th.