Dousaibou Seibutsu.

Nakagawa and Yokota are two boys very much in love with each other. Yokota is more open with his feelings, but Nakagawa is easily embarrassed of public displays of affection. Though they may be outwardly different, their feelings for each other are the same and their connection is such that they liken themselves to Dousaibou Seibutsu (Same-Cell Organisms).

(Source: DMP)

Contains the Stories:
1. Same Cell Organism
2. I Love You
3. Lullaby in My Hand
4. The Letter in the Attic
5. To Make Angel (Tenshi wo Tsukuru) (Prequel)
6. To Make Angel (Tenshi wo Tsukuru) (Sequel)
7. We Selfish Two

(Source: MU)