If the two have similar Sword Realms and their insights are aligned, then naturally and without effort, they will ascend to become a Third-Turn Sword Immortal.
However, if their Sword Realms differ too greatly—meaning that the Sword Intent they grasped before attaining immortality is completely at odds—it will cause a conflict within the Sword Realm. If one cannot completely suppress the Sword Realm contained within that Sword Bone, not only will ascension be impossible, but it will also damage one’s own Sword Realm.
"The Spirit Dragon Constellation… it seems the Ning clan is located there."
Chapter 624