In a song I'm in the middle of tabbing the guitarist plays a natural harmonic at the 14th fret on E and the 15th on B by fingering 2 on E and 3 on B and picking with his thumb while covering the frets at the top of the neck with his index finger to make the harmonic. (I hope that makes sense)

Just use the natural harmonic symbol on whatever fret and explain the rest in the tab, thats how i'd do it.