Both are traditional singing and instrument styles. Both use deep registers. Both are soul shivering harmony. Why haven’t they collaborated yet? It could be one of the most metal natural sounds song or (fingers crossed) album. Can you imagine the energy that would come from it?
AFAIK, Heilung shouldn’t be considered traditional music. Many elements of their music are borrowed from unrelated cultures. Throat singing for example is not a Scandinavian tradition. They also use Hindu instruments in their compositions. Heilung themselves don’t even describe their music as traditional, yet they have some rabid fans that try to shoehorn their work into scandinavian tradition, muddying the historical waters for anyone unfamiliar with scandinavian tradition. Cool band, but yeah. Odd dynamic going on here.
Still, I bet a collaboration would be cool.