so is this a BL or

It's BL. Xiao Fu and Mr.Zhen had some kind of phenomena through which their bodies have to take turns because both can't appear at the same time anymore, and thus their bodies transform into the other's randomly. This gets the CP into a situation in which they are obliged to cohabit at the same house and get to know each other. In case you're worried, there's no cheating on the wife, you'll know what I mean if you read it.

It's s bl, it becomes more obvious in the later chapters when their bodies go back to normal