Originally Posted by: kvsealegs

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.

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Yes, that's called playing in harmony. You've got the basic idea right. One guitarist plays a melody & another guitarist plays a harmony of that melody. Usually a 3rd, sometimes another interval.

Guitar 1 plays scale degrees: 1-2-3-4 and so on & at the same time

Guitar 2 plays scale degrees: 3-4-5-6 and so on

I cover that topic in detail in a series of tutorials on harmony guitar playing. Here's the first one.

https://www.guitartricks.com/tutorial.php?input=1965

Originally Posted by: kvsealegs

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?

Sure, that's also done. Usually if there is more than on guitarist in a band they play complimentary parts, slightly different voicings of the same chords.

Guitar 1 might play a C chord here:

|--------------------------------------------|[br]|---1-1---1-1---1-------------------------|[br]|---0-0---0-0---0-------------------------|[br]|---2-2---2-2---2-------------------------|[br]|---3-3---3-3---3-------------------------|[br]|--------------------------------------------|

While guitar 2 plays here:

|---3-3---3-3---3-------------------------|[br]|---5-5---5-5---5-------------------------|[br]|---5-5---5-5---5-------------------------|[br]|--------------------------------------------|[br]|--------------------------------------------|[br]|--------------------------------------------|

[quote=kvsealegs]

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?

That's not how it works. :)

The entire idea of a relative minor key relies on using the same key (same notes, pitches, chords), but starting on a differen scale degree.

C major scale is:

c-d-e-f-g-a-b-c

The relative minor of C major is A minor. This is because it uses the exact same notes, but starting on the 6th scale degree.

a-b-c-d-e-f-g-a

So, retuning your guitar doesn't accomplish anything toward playing relative minor. It will even make it harder to figure how how to do it since the notes in absolute pitch will be in different places on the 2 guitars that are tuned differently. :)

If you wanted to play in harmony but attempted to tune one guitar down a third you wouldn't get diatonic 3rds, you'd get parallel 3rds which won't work for traditional harmony. You will get an interesting sound. But not diatonic harmony or relative minor.

Hope this helps!