to get back to the OP, most english and irish fiddlers employ or use double stops of a fifth on long notes[
technically that is a harmony note and octaves on lomg notes
in sliabh luchra [ northcork kerry south east limerick] there is a style of octaving on melody, when two fiddles play together, where one player plays the melody 8 notes down, not a harmony, but very effective see julia clifford and denis murphy