This may be a bit off the mark, as it isn't necessarily a duo, but you could be:
Jason and the Argonauts.
One the voice of a city in italy, the other a taker of fleece.
The taker of fleece that I am thinking of is Jason, who quested for the Golden Fleece with his team of Argonauts. In Italy, there is a municipality called San Paolo d'Argon.
Together they make a duo.
Again, not the most straightforward duo, but this is a cryptic puzzle, so nobody is saying it should be straightforward. Anyway, one and another are a duo; in this case, Jason is one and the Argonauts are another.
Like calcium to cobalt.
Who rarely appear outside of their element.
This seems to point to the element Argon on the periodic table, but there is likely more than I am acknowledging.
But again, this duo may in fact be
Jason and the Argonauts