Why go to two guitars?
Well, the big difference for me is that I can stop playing and really sing. It's hard to focus on two things at once and some of my songs really require concentration to hit the runs or hit and maintain high pitches. So guitar #2 covers that main parts of the song so everything stays sounding full. That's just whats in it for me.
On another level, when you get two guitar players schooled in old Muddy Waters, there's a great deal of counter point and dissonance going on. You can't fake that with a single guitar. A guitar and a keyboard can get you close. Two guitars are magic.
Also, there are musical parts that can only be played with one guitar player in standard tuning and another in say, open G. The chords sound different because of the intervals involved.
Oh, playing lap-steel in a trio is fine. But having a second guitar player there opens up more keys for me where the chord voicings aren't as full or are a little odd. So, blues in F#minor or B minor on a steel guitar is totally cool with a second player. AND, I get to play harp too. AND, I think having a second guitar player makes you work harder too.
OK, so it's all about me from the sounds of it. Well, the music is different in a good way, that's really what's important.
