From actually knowing the relation, it would be kind of odd if the dad did have an affair since we know for a fact that Namsoo's parents are dead. There's no hint that the dad knows of Namsoo or cares for him.
They are twins. Hyeongoh and Namsoo are orphans. They were also separated on the orphanage
Where were you able to get this information?!
Starting in chapter 54, we get a flashback that explains everything.
Where did you read chapter 54?
It's raw, but I read it here:
https://toonkor.com/%EA%B5%AC%EC%9B%90%ED%95%98%EC%86%8C%EC%84%9C
Chapter 60 just updated today.
The next few chapters we might finally see the root of all the problems. Also with the new revelation that Namsoo and Hyeongoh have some similarities has got me thinking that maybe the dad had an affair of some kind, they are some how related, or they got some Prince and the Pauper shit going on.