My wife plays viola in a local symphony and I always take her to the evening rehearsals. When she and I play together it's her on the viola and me on a 12-string tuned 1 1/2 steps below concert pitch. So, she does both classical and folk. She and I have observed that the differences between fiddlers and violinist is that (1) the fiddlers rarely get out of 1st position and use open strings a lot; (2) fiddlers use their bowing hand more than violinists, who have more technique in their fingerboard hand. I don't know about triple-stops, but there are plenty of double-stops in classical music.