yachiru unohana

If all Kenpachi's came from killing the previous title holder of the name Kenpachi, how did the second Kenpachi become to be if the first one is still alive during Zaraki's time as the 11th Kenpachi?

As the title suggests- I am wondering Where is the ART in Zakarai's Training? the point of this training was to teach him the Art of killing. Unohana is killing him and reviving him until he reaches his unsealed former state that he had when he was a kid and could have killed her...