They were simultaneous, but in different worlds -- Laura in urban NYC, Joni in Canada and California.
Really, they are very different, but they both had that soprano capability. Laura wrote everything on piano - Joni only occasionally wrote on piano.
The other thing about Joni. She defined herself as an artist/painter first. The folk-singing she did for the money, she has said. It was a means to an end initially. Laura, I suspect, had an irrepressible passion for the music.
While Laura was doing the above song, Joni was doing this one (very light-hearted and conventional in comparison):
http://www.youtube.com/v/F_y7O06z77QI'm not disrespecting Joni, because she was constantly growing and changing. In fact, her career continued consistently, whereas Laura did semi-retire until she died (of cancer).