I still don't understand why they're constantly switching places like that
thanks for the TL
We don't really know other than this is what Zhou (the woman who's assumed the role of emperor) wants.
My guess is she wants a body double just in case (it's a common trope in this kind of setting). A body double can serve as extra security if there's an assassination attempt, with the double being targeted rather than the original person. Also, once he's properly trained, the double can gather intelligence acting as the emperor while Zhou is elsewhere gathering intelligence as a concubine.
In addition, Zhou probably enjoys forcing a man to repeatedly become a woman and act the role of a concubine.