I'm pretty sure this was done just for sonic purposes.

About the talkbox thing -
You only need one mic for a talkbox, the signal usually comes through a tube from a horn driver that's attached. The horn is powered by a low wattage amp that the guitar is fed to. The tube then goes into the guitarists mouth where the sound is manipulated by them moving their mouth. The mic is only there to reproduce, or reinforce this.

Maybe you were confused by that notion? Not sure.

Anyway, I remember seeing photo's like this.. Cream, Hendrix etc. and I was always told it was just to add the 2 signals together for better, or different, sound.

seen a lot of jimi hendrix footage using 2 vocal mics... don't know why though, just sayin I seen it.