J. McGuinn & R. McGuinn are the same person, now known as James Roger McGuinn (born James Joseph McGuinn III on July 13, 1942).
At the start of his career he was known as Jim. The story goes that he changed his name in 1967 because a guru in Indonesia said that a new name would vibrate better with the universe. The guru sent Jim the letter "R" and asked him to send back ten names starting with that letter.
Because Jim was into gadgets and Sci-Fi, he sent names like "Rocket" and "Ramjet." He included the name Roger only because they use it for radio messages to indicate "OKAY."
Roger was the only "real" name in the bunch and the guru picked it. Jim only changed his middle name from Joseph to Roger but used Roger as a stage name.
As far as the vibrating with the universe ... he didn't notice any change and would have changed his name back to Jim (which he liked) but that would have been too confusing.