Ohhhohoho, I just had a Thought.
Let's assume that sword spirits in SVSSS are something like Bleach. Ideally, you get a perfectly blank sword out of the sword wall, and it'll be a perfect match for your current physically abilities, but also grow with you to stay in that "perfect for you" range. At the same time, it starts off with a spirit inside that's little more than a blank paper doll, but as you work with it and grow and really make the sword your own, it starts to- for lack of better words- become you. Or, maybe become like you? For example, when a disciple finally takes on a sword, they'd eventually see it come to adopt a similar physical appearance to themselves. At the same time, the thing in that disciple's heart that they hold most important and dear becomes that Swords defining feature- owner values family, sword values family. Owner values strength, sword values strength.
Their personalities, however, can sometimes widely range. Say the disciple in the example is a calm, pleasant person, but they (and their sword) values...idk, perfection. Their sword could also be perfectly calm, but pleasant might be out the window in favor of demanding, well, perfection. Mastery without flaws, even in themselves. In this way, Swords can sometimes be exactly like their wielders, or sometimes completely different.
(For a more close to home example, in PIDW, Ning Yingying’s sword could be described as her greatest critic; often frustrated and angry when spoken to. Ning Yingying’s naivety and constant accidental trouble making for the people around her likely greatly irritated her sword, who could plainly see in a third party view where things were going wrong, but could not get Yingying to listen or change- and in PIDW their relationship either likely never improved, or did so too late.)
In PIDW, Shen Yuan noted that, once Luo Binghe received Zheng Yang, the sword very quickly took on both Binghe's personality and appearance. He theorized this was because Binghe, despite everything, was blatantly himself, never once losing sight of the things he valued even as everyone else treated him terribly, and since Binghe and his sword both appreciated kindness and were never cruel to each other, they quickly forged a strong bond. (Shen Yuan would later quote a scene often in the comment section about how, when Zheng Yang was confronted with his owner's mixed heritage, the sword didn't so much as flinch, saying that his wielder was his wielder to Binghe's face, and that he would have no other. Shen Yuan would cite it as one of Airplane's best written moments, finally giving Binghe someone who loves and cares for him genuinely. When Zheng Yang is taken from him, Shen Yuan probably screams. Like, outloud.)
Shen Yuan would also note that, from what little hints they were given throughout the narrative, this was maybe not the case for Shen Qingqiu and Xiu Ya. Details were scarce, but theories abound that even Xiu Ya couldn't stand that man's ass.
When Shen Yuan Becomes Shen Qingqiu, this theory is thrown out the window. The reality of the matter is, when Shen Jiu pulled Xiu Ya from the sword wall, his values were simple: Survive.
In PIDW, it was often noted that in moments Xiu Ya was mentioned, it was often just a short sentence that read, "Xiu Ya rattled in its sheath." Well, it turns out Xiu Ya was rattling because it was constantly trying to get Shen Jiu to stop sabotaging himself! And now, here in this world, Shen Yuan has to deal with an angry, depressed sword who's just lost its original owner forever.
Even Shen Yuan's weird divorce from reality can't withstand the power of a sentient Sword's grief. It leads to changes- specifically, changes in how Shen Yuan sees the world. Gone is that brief mindset of everything being a book! Now there is only the terrible bleak reality that he, Shen Yuan, is now in charge of so many children and the fate of the world, basically.
Also a traumatized sword who demands he take care of things proactively and NOT die at the hands of system, thank you very much.
(Oh yeah, Xiu Ya knows about the system. Kinda hard not to. They get into arguments all the time. Shen Yuan would laugh, but considering they often argue about things that would lead to his eventual, painful pickle pot death- well. It's less funny.)
Even to System, it's a bit surprising how quickly Xiu Ya latches onto Shen Yuan. For a sword confronted with a whole new person replacing his original wielder, Xiu Ya gets over his anger rather quickly. Shen Yuan knows Xiu Ya would well be in its rights to resent him and simply seal itself in its sheath forever, but instead, it switches over to hyperviligence very soon after the Skinner Mission.
What Shen Yuan doesn't know is that, despite how outwardly different they are, Shen Yuan and Shen Jiu are remarkably similar in Xiu Ya's eyes. Both are intelligent, both are constantly afraid, and most importantly, both want to survive. The only difference it sees is that of Flight vs Fight, and their odd attitude towards one half-demon child.
This is it's second chance. Xiu Ya will not lose another wielder.
After the IAC and the Endless Abyss, Shen Yuan gathers up Zheng Yang's shards and has it reforged. He plans to make a sword mound, but...he just can't help himself. Xiu Ya calls him foolishly sentimental, but he has to know. He has to see if Zheng Yang- and thus the last of his white lotus- is really gone. He sends out a thread of qi into the blade.
He sighs and pulls away. It was a long shot anyway- but Xiu Ya stops him. Tells him to try again. He argues that it's pointless, but Xiu Ya is insistent.
"Like I said, there's nothing-"
He nearly drops the blade. Xiu Ya is smug in its sheath. Zheng Yang- it's not gone!
The next year is dedicated to the younger blade's care. It grows back it's strength slowly, and though Xiu Ya is standoffish at first, it soon becomes just as involved in the other blade's care as Shen Yuan himself. (Perhaps Xiu Ya was adapting too well to him.)
Well. There comes a point where Zheng Yang becomes aware again. At first it refuses to speak directly to Shen Yuan, not out of malice or anger, but out of propriety. Swords do not speak through words, they speak through qi connections; and since forging that connection is deeply personal, speaking to Shen Yuan directly....well.
That changes once it realizes Binghe is gone. Or, perhaps more correctly stated, *Xiu Ya* changes that once Zheng Yang realizes Binghe is gone.
It's not entirely unusual for a cultivator to take up two sword spirits and their corresponding blades at once. There is, in fact, precident for it, despite what Shen Yuan may think. Xiu Ya just neglects to tell him that such things are usually the result of a cultivator losing someone close to them; usually close family, but more tragically, a dear person most beloved. Like, say, oh, I don't know...a lover? [Smug sword emoji]
Why did Xiu Ya encourage this? Because over the year of helping care for it, Xiu Ya gained a soft spot for Zheng Yang. It could sympathize with the younger blade's plight, and if anyone could understand the ache of a bond broken, it's Xiu Ya. So, why not take out two birds with one stone? Zheng Yang needs a new wielder, and Shen Yuan desperately needs his white lotus to keep him going. (Besides, unlike PIDW Zhang Yang, this version of the sword was made from Binghe's love for his Shizun. If Shen Yuan can't be his new wielder, then who else could possibly take Binghe's place? No one. No one at all.)
Shen Yuan thinks this is a terrible idea, but once they all meet in a dreamscape and he sees Zheng Yang's pleading eyes in person, he caves. Those are Binghe's eyes, after all.
Still, he can't help himself. He asks Zheng Yang if it really wants to bond with a man who threw his disciple into the Abyss; but Zheng Yang is no fool. Xiu Ya has told it of System, has spoken of Shen Yuan's true identity and his plight, and his terrible regret and guilt for not being able to stop it. Zheng Yang is also Binghe in all but name, formed from Luo Binghe’s determination and dedication to this very man. Shen Yuan can try to sabotage himself all he wants, but Zheng Yang will not allow this beloved, silly man to deny himself what they both want.
Thus, Shen Yuan becomes known as the Dual Wielding Widow of Qing Jing Peak.