How do you play two guitars in harmony?
When you harmonize two guitars you play one thing say at the third fret( Key of G ). Then you could play the same thing say four frets up but if the notes fall outside G, then you alter them by playing a fret higher or lower for any given note. Look at first two bars of that tab. Both guitars are playing in A.
Why do bands use multiple guitars?
So, why do bands have two guitarists? Bands have two guitarists in order to thicken up their sound, maintain momentum when switching from rhythm to lead, perform layered parts and add a sense of variety to their sound.