I'm with Pavane here; the original Carthy/Swarb albums were ground-breakers; IMO they "shaped" the "Folk Revival" in the UK. AND the two guys have been working together ever since.
Steeleye and Fairport may be "classic" (although that's not my opinion of them) - but Carthy has not had a strong direct influence on them; nor did he on the Watersons. Not sure about Swarbs creative input to Fairport; I suspect that's probably rather stronger.
To me they are the obvious "Collaboration" of on-going UK folk history.
But let's broaden this . . What other "Folk Collaborations" would you nominate as particularly significant?
I'll go fo:
Ewan MacColl and Peggy Seeger June Tabor and Martin Simpson.
G.