Here's an interesting artwork (above) depicting Paul Simon and Art Garfunkel sharing a single microphone for their track The Dangling Conversation. Clearly it's just an artwork and it may not be a depiction of anything that happened in reality. Or the artist might have been working from a photo, so it could be the real set-up.
Should two singers share the same mic?
The obvious question is why were they not given a mic each? Surely the studio had enough mics?
The answer to this is that when two singers blend naturally, it doesn't make much sense to separate them out onto individual tracks then try to blend them back together again. It would be undoing the years of experience PS and AG have in making their voices work together. This applies to any group of singers that blends well naturally. You'll be making life a lot harder for yourself if you give them individual mics, unless there is a good reason for doing so, like live performance or broadcast.
Should Paul Simon have stood on a box?
This is the next obvious question. In photos, Art Garfunkel is clearly taller than Paul Simon, probably to a greater extent than is illustrated here. According to the illustration, Simon is about twice as far from the mic as Garfunkel. Inevitably therefore, Garfunkel will be louder.
But hang on… Garfunkel is the better singer, so perhaps this is how things should be. But I doubt it. Here is the one photo Google's ambition to catalog the whole of human knowledge can provide of S&G working together in studio conditions…
What I read into this is that since Simon is singing without headphones, presumably hearing live musicians, Garfunkel is doing the balancing. Perhaps I'm overthinking the issue, but it is true that musicians react to what they hear in their headphones, and the engineer should pay as much attention to this as to any other aspect of the recording process.
You might also notice that Garfunkel is watching Simon. Any performers playing together will do this. Simon is the boss of this duo, so he's the one who gets watched here.
In summary, if any group of musicians sounds great playing together, there is no reason to split them up onto separate tracks. The resulting mix will certainly be more difficult, and probably not as good as the original blend.