Part of my confusion on the key was that there are some masterful guitarists that play rifts together in unison, some times like an octave up and I guess you can also do it in degrees. Probably most often 3rd or 7th? It would take a pretty good grasp of the scale, but I'm guessing that is the way it's done. The Allman Brothers Band, and The Outlaws come to mind as using this frequently.

I was hoping that two guitars could strum the same chord positions and due to the same consistency of notes playing in the chord it would be complimentary. For example an open C and a barre C sound quite different when played in unison, even though they are both the C chord. I am not certain if that is because of the order in which they are strummed or more because of a difference in timbre?

This last question I am sure will floor you with my lack of understanding of practical music theory. But I will ask anyway. Every key has a relative minor. If a second guitar was tuned in a relative minor, could two guitars play open chords together? Are there any harmonic keys?