While we're at it, I should just reinforce the point made above that substitution of Mary and Flo was in any case the exception rather than the rule. I maintain that, on the rare occasions when it did happen, this was more or less the way of it. As an exercise, anyone out there who can sing, have a go at learning the backing vocals to Forever Came Today and Love Child. You don't even need to invent them in the first place. Just be honest about how long it takes you to get all the harmony parts down.