In these pictures it's likely that one mic is for the PA and the other mic is for recording. Either they didn't have mic splitters or they didn't trust them! This was a very common way of doing things in the 70s.
The Grateful Dead are known to use two mics as a noise cancelling technique. The output of the two mics is combined with equal levels but opposite polarity.
In this arrangement two omnidirectional mics are set up so that one is a few inches further from the wanted sound source than the other. That's quite a different physical arrangement to the one in your photos.
The signals from the two mics contain almost identical levels of background noise, but one has a louder version of the vocal than the other. By combining the two, with reversed phase, (background noise minus background noise = silence), (vocal from closer mic minus vocal from further mic = audible vocal). There is some phasing-related distortion on the vocal, but in a live setting it's too subtle to matter.
Of course this doesn't only apply to vocals - it could work for any sound source. Indeed it may be more suited to acoustic instruments, because it's reasonably easy to shove your mouth right up to a unidirectional mic -- it's less easy with certain instruments.
Most bands don't need this because they play on a reasonably quiet stage, with the PA speakers in front of them. The Grateful Dead play in front of a loud backline, so they need tricks like this to deal with background sound.
I can't find any photographs of the Grateful Dead employing this technique - so it's possible they only use(d) it in certain circumstances. All the photos I've found show them in the 70s, with one mic for PA and one for recording.
Different microphones do have different characteristics, and it's certainly possible to fine-tune the sound by combining signals from more than one mic - perhaps running each one through a different effects chain.
However this is something you'd be more likely to do in a studio than in a live setting.
I have seen footage of Bryan Ferry performing with two handheld mics, deliberately moving one or the other closer to his mouth, to achieve a stereo pan.
During Live Aid, Bryan Ferry was apparently using two mics for musical effect. In fact, one was broken:
"And then my microphone wasn't working, which for a singer is a bit of a handicap. A roadie ran on with another mic so then I was holding two mics taped together and I wasn't really sure which one to sing into. It was a great day though."