CookingPriest said:
Not without precedent.
Ichigo got two swords because of his dual heritage(one for quincy/OldMan one for shinigami/HichigoZangetsu), while Ukitake had a separate being inside him.
So far all the twin sword users before had some special circumstance.
Saw a post from reddit that was quite intriguing, it implied that Nanao's mother was Kyouraku's twin sister, that would actually be cool giving the fact of how similar both Zanpakutos are and that would also put her in a different level of respect for him, hence why he's alway taking care/using her kimono(if that is hers).
All that and plus some more details of how he can use both.