Originally Posted by: schmangeOr it could refer to tapping two strings with the right hand to get a harmony.

I agree because -

1) Schmange knows a considerable amount about guitars and playing them

2) I've actually also seen websites where this guy uses double tapping to do piano arrangements - he'd get the bass part going by tapping with the left hand at the bottom of the neck on the low strings, then tap chords with his right hand on multiple strings... (*sick*)