Never seen a Slovakian version of that - I have Hungarian/Romanian double whistles made by Gyorgy Ban in Budapest, one in D and one in A, but yours sounds like quite a different beast. What pitch do you mean by "low G", the G above or below middle C?

Off to Gyimes tomorrow where I expect to see some Hungarian instrument makers showing their stuff. The Csango shepherd's flute is a reverse conical bore instrument along the same lines as the Sweetone but made of wood; works very well in the extreme high register. I have an A (the usual pitch) and two different B flats but I could do with a C and a G.