Well Barbara is a great player. Actually, she can do it mainly because of the notes system of her Mark Ramser hackbrett : it's made so that the space between different intervals is the same all along the courses : same space for thirds, same for fourths, and so on. And you can play the basses on either side of the instrument. I tried with my traditional tuning, but it's really too difficult for the head and the hands...
of course that doesn't alter the huge qulaity of Barbara as an Hackbrett Player : she plays a lot of improvisation music, latin american, on both the Ramser and a traditional hackbrett, with traditional tuning... (but I saw her playing with four hammers only on the Ramser...)
Glenn McClure also plays (played) in four-hammer style. Haven't actually heard anything by him, but apparently he was the first to introduce this technique inspires by the four-mallet vibes player Gary Burton.
Here's a quote from Andy Fagan being interviewed in 1986 -I'm not a player now compared to 50 years ago, and I'm not boasting when I say that, but I could play it in those days with two sticks in each hand, one stick harmony and one stick melody