It is also possible that sometimes, a Nefesh of a Tzaddik can come into him (via Ibur), and afterward [through his additional good deeds], another Nefesh of a Tzaddik which is on a higher level from the first can enter into him (via Ibur). How this works is that the Nefesh of the first Tzaddik comes in [in the 'slot'] of where the Ruach would be, and the second Nefesh from the other (higher) Tzaddik would enter [in the 'slot'] where the Neshama would be.
Also sometimes it happens, that after a person reincarnates with all three parts of his soul (Nefesh, Ruach, and Neshama), he rectifies them all (and he is still alive) -- a Nefesh or Ruach of a Tzaddik [can come into him in the secret of an Ibur, and that Tzaddik would help him and raise him to a higher level and complete his Nefesh, Ruach, and Neshama]. Then when this person leaves this world, he is able to elevate to the level of the Tzaddik who entered into him (via Ibur), and there in the Olam Haba, the two of them will be together on one level. This is the secret of what is written at the beginning of Bereishit in Sefer haZohar, page 7:2, that haRav Shimon ben Yochai (alav haShalom) fell on his face, and he saw Rav Hamnuna Saba (zichron l'vracha). He [Rav Hamnuna Saba z'l] said to him in the Olam Haba that they would be together as neighbors -- he and Rav Hamnuna Saba [because he entered Rav Shimon ben Yochai as an ibur during his lifetime].