According to the Internet source given below, these global artists, Nanne Kalma and Ankie van der Meer, became a folk duo in 1980. Since then they have produced more than 30 albums (LP's and CD's) and are still counting. They sing in Esperanto, in Frisian, in Dutch and in English and perform using guitar, mandolin and concertina amongst other instruments. They appear to love touring and have traveled widely.
On a personal note, I had the very good fortune of meeting them at the Bridge Folk Club
in Newcastle Upon Tyne in March, 2011. I was much taken by the similarity between (Dutch) Frisian and Platdüütsch and had the opportunity of singing Dat du mien Leefsten büst
to them to which they replied with their version of the same song but in Dutch(Frisian). A lovely moment! As a memento, I took home and prize their atographed CD, Overwinteren
: Courtesy Nanne & Ankie.