Frank Heim and Stefan know each other for about 21 years since both played on a stick compilation album in 2001. Frank aka Bruder Frank (Saltatio Mortis) is playing mainly bass in one of the biggest medieval metal groups in Germany called Saltatio Mortis. He just switched to Touch Guitars® and will be on stage with his U8 on the upcoming tour, too.
Frank’s Artist page: https://www.touchguitars.com/frank-heim/
Here is a video of a live performance where he plays the U8 as a regular bass: