Just saw this thread. Can't believe that nobody has yet mentioned (unless I missed it) the Original Carter Family. Gloux's post recommended Kay (Justice) and Ginny (Hawker) with which I agree, but you oughta hear the people they learned it from!

Also the old brother duets (Louvins, Delmores, Blue Sky Boys, Monroe Bros) and their non-sibling precursors (Karl and Harty, Mac and Bob)