Agree with those who mentioned Kay Justice & Ginny Hawker – their harmonies give me shivers, they're so good! (Tried to find a link for the two of 'em, but the best I can do is this page of a website for Ginny Hawker and Tracy Schwarz (if you scroll down, you'll get to their CD "Bristol, A Tribute to the Carter Family")

Also forgot to mention John Roberts & Tony Barrand, English ex-pat (I think they live in New England) harmony singers whose voices combine brilliantly.