One soul, but the previous incarnation Yonosuke, after dying and being reborn, still had his consciousness instead of naturally integrating into the personality of the new incarnation. So there were basically two separate identities, what were still the same body and soul.
When Yonosuke said goodbye, he got the closure he never had before, and so his consciousness faded, and went to what it originally was meant to be, where his identity slept in Ryoichi's subconscious.
And yes but no, because Yonosuke birthed Keisetsu, and he is his mother. Ryoichi is the same soul, but a different person. So he's his mother, and he isn't, at the same time. They're still family, though.
Is Ryoichi and Yonosuke the same entity with different personality?
Is Ryoichi still considered as biological Mother of Keisetsu?