Early Japanese definitions of the mitama, developed later by many thinkers like Motoori Norinaga, maintain it consists of several "souls", relatively independent one from the other. The most developed is the ichirei shikon (一霊四魂?), a Shinto theory according to which the spirit (霊魂 reikon?) of both kami and human beings consists of one spirit and four souls.
As the quote above notes, the "four souls" construction applies to both kami and humans. How it applies in this or that individual's practice would depend greatly on whether the practitioner was working solely to unify his or her own "four souls," or to unify his or her own "four souls" with those of one or more deities, and if the latter, the particular attributes of those deities which the practitioner seeks to invoke. At its most extreme, the practice is used precisely to induce spirit possession, much in the manner of the Vodun practice of drawing down the loa to "ride" the worshipper. For a fuller discussion of chinkon-kishin in the context of Omoto practice, see Chinkon Kishin: Mediated Spirit Possession in Japanese New Religions
, By Birgit Staemmler.
This construct is distinct from the Chinese In/Yo filter; Ueshiba used both, which would seem to suggest that he felt each had distinct strengths and weaknesses as principles by which either understanding or training might be ordered. One crude analogy would be that of the use of two lens filters on a camera or other optical instrument, each of which makes different aspects of the same scene visible.
For my own part, I'm quite skeptical of any effort to develop a quasi-mathematical relationship between In/Yo doctrine and Ichirei Shikon doctrine in which the four factors down neatly to the two, not that there's been any shortage of attempts to assert such a relationship over the past couple of centuries.....