Actually you dont need to read the one before. Yeah there are some parts where characters appear and situations that may be like who's that, they work together, since when? But for me it's minor, as story focuses on Yuta and Seunghyun and their story. If you know the main characters from first series Dooshik and his man (forgot name, haha) it's okay as till now, they appear the most.
I didn't read the first and I'm not confused as much, I dont wanna read first as I saw the abuse and etc, not my cup of coffee. (There's too, but both are red flag and they abuse "each other", so no other person raping or hitting, etc)

I'm at 15 ch. Is it worth reading?
I won't read the other series.