A few weeks ago while attending the Swannanoa Gathering's dulcimer week, I had the opportunity to spend a wonderful and fascinating evening by a pond listening to and watching Ken Bloom play, among other things, the concert zither. While demonstrating the zither, he played "The Third Man" theme a couple of times. Yes, there were only two hands and one zither. Yes, it sounded just like the movie soundtrack. And yes, both Mr. Karras and Mr. Bloom are (IMHO) geniuses.
I can't speak for mr. Karras, but Mr. Bloom is also very funny and one of the most genuinely nice people I've had the pleasure to meet as well. I'd highly recommend that anyone interested in old-time music pick up a copy of his CD, That Banjo From Hell which includes no zither, but does include the banjo fiddle he built. It's definitely a different sound.
Along with the concert zither and a strange Ukranian instrument the name of which I can't recall, Ken also sang and played some Civil War songs on the Breton small pipes. Quite an experience!
And don't get me started on the violin/hurdy-gurdy-like Nickleharpa I heard later on that evening! Beautiful!