political reasons - 1. no one knows about a stand in, it'd seem like he's banishing his pregnant wife for a mistress from a powerful family, oh maybe they want power and are using her to manipulate him, heavens he's a puppet emperor wielded by the cheon family, time to depose him, appoint someone as regent and crown the baby in the emperess's womb as the next emperor. (one that will be a puppet at the hands of the emperess's family)
2. if he admits to the usage of a stand in, he loses a shit ton of credibility, the legitimacy of both children arises, ppl will be like is he the one who was ruling or was it the stand in, the noble families of his wives will riot and he'd get deposed as an immoral emperor
why cant the emperor just banish the empress,,, like ban her from the palace to another far away place or something…