haha well, if you're describing a scenario where the universe decides for you, and everyone knows who their assigned soulmate is... imo, soobin would dismiss the entire system lol. didn't he once mention he actually chose his soulmate every year back in school? even now, he chooses his own in beomgyu. soobin strikes me as the sort of person who puts his own intuitions above the whims of the universe. he's likely not to care too much about assigned soulmates—he'd find his own. he'd be curious about his true soulmate, of course, and he might give them a chance like everyone else, but if he found out he didn't love them there's no stopping him from disrespecting what the universe wants for him lol.

on the other hand, the romantic in yeonjun might start off regarding true soulmates through a rose-tinted lens. he'd be shy, go with the flow, open to it going somewhere—and give his best in hopes of unlocking the magic that made them fated soulmates in the first place. but he's also not one to waste any time. if he just doesn't feel the spark/connection, or he doesn't admire them in some way, he'd eventually become just as dismissive about the system as soobin is. he would probably be more quiet about things not working out, though.

in the case that ynbn are true soulmates, the funny thing is that neither of them would believe it. over time, i think they'd realise they're more similar than they think, and they are called to each other's lives to make the other better. hahaha.

for the record, i'm not a soulmate believer haha. be it irl or as a fic trope, the concept itself doesn't really excite me... but i'm curious! how would you answer your own question, anon? :)