I know what you mean, reubent. Prine is a fine songwriter, and a singer with his own identifiable style and sound, rough as it is. But he, like Johnny Cash, Willie Nelson, and Bob Dylan, is really difficult to sing harmony with. His phrasing is just so unpredictable and erratic, that staying in synch with him is impossible!
If you're lucky, John's pal Iris Dement may appear with him as a guest. They often share the stage with each other.