That cut was probably taken from the Bellamy compilation Wake the Vaulted Echoes--that same picture is used on the front cover. The track was originally released on Both Sides Then, and the other singer was indeed Louis Killen.

For other examples of this kind of harmony, check out these classic pairings: Lou and Sally Killen, John Roberts and Tony Barrand, Ian Robb and Margaret Cristl, Maddy Prior and June Tabor. The Coppers and the Watersons also did a fair number of duet harmonies. Peter Bellamy wrote some nice duets for Mike and Norma Waterson in his ballad opera The Transports.