The idea that singers might have been members of church choirs is certainly true of Joseph Taylor, and it might have helped to contribute to the quality of his voice. Could he harmonize spontaneously? I don't think we know.

The 'communal harmony singing' question has come up on Mudcat before; I did a bit of listening to the recordings I had here of singing pubs, and wasn't able to make out much harmony. OTH, my 'Wild Rover' researches turned up a version from the songbook of Thomas Hardy's father, which was scored for two parts.