I was trained as a classical violinist and self-trained as a fiddler. Put one finger on two strings. Fifths occur frequently in violin/fiddle music of all kinds: the opening bars of Swallowtail Jig, Morrisson's Jig, the B part of Devil's Dream or De'il among the Tailors, works by Bach and Mozart, etc. It's also the only way to play fifths as double stops, and they make great ornaments.