I was confused too. I play cello and a harmonic to me has always been the softer octave that plays when you touch the middle of an open string. If you bow it and release your hand it slowly fades into the lower octave, but you can't play both at once. I guess if the sound was still in the air you could play both.

It's absolutely true : when you pluck a string on a guitar (or fiddle, or banjo or piano...) the string is emitting simultaneously all the possible harmonics : in fact, it is a multimodal emitter. BUT not all the harmonics are emitted with the same amplitude (or energy) ; in most cases, the harmonic #1 (called fundamental in Europe) is the most energetic of all and the energy decreases with the harmonic rank. The resonator (body of the instrument) selects some of the harmonics to be more amplified than others : that's the reason why guitars of different brands don't sound the same ; the most illustrative of my statement is given by resophonic guitars (Nationals or Dobros) : they over amplify some harmonics, (around the 1500-3000 Hz range), because of the presence of the aluminum cone inside the body... A guitar is a multimodal resonator, with a variable gain (amplification or damping) for each frequency ; the guitar player also does the same, when he changes the place he plucks the string (close to the bridge or close to the neck) : the percentage of higher harmonics will be modified ! S Ceyral (ret. Associate. Prof of Physics)