The process of breaking through the Core and Soul Fusion Realm was already on the name itself. One had to fuse one's Soul and Core inside one's dantian together. Of course, it wasn't the entire Soul, but just a small piece of it.
[Simply put, you guys will be cutting off a small part of your Soul and feeding it to the Core. Now that you two are at the peak of the Core Formation Realm, the Core has reached the necessary requirements to hold and nurture that piece of Soul.] Sister Orb said as Rean and Roan began their breakthrough.
Rean and Roan understood why as one of them replied, 'The Soul being nurtured will then grow all the way to the peak of the Core and Soul Fusion Realm. Once we reach that level, it will give birth to a Nascent Soul. Is that correct?'