Love is to Understand Love (Koi wa Koi o Shiru)

From Renta!:

"...If you're OK with my wife being my number one..." Masayuki is a businessman who's having an affair with his boss, Mr. Shimazu. Their love is a secret so well-kept, they can't leave even a shred of evidence. One day, a younger man, Minami, says to him, "You're going out with Mr. Shimazu, huh?" With those dangerous, half-threatening words, Minami comes to live with Masayuki...

From [Attractive Fascinante](https://www.mangaupdates.com/groups.html?id=681):

Shouji was upset when Shimazu cancelled their appointment. Minami approached him and told Shouji that he knew that Shouji was having an affair with Shimazu. Shouji denied it and Minami asked Shouji to let him stay over at his house for the time being. With Minami stepping into his life, would Shouji finally let go of his tiring love with Shimazu and accept Minami?