Unmei ni Kiss

From Storm in Heaven:
What happens when your mother decides with her best friend that their two children will be soulmates? If the two children turn out to be boys, nothing, right? It seems fate was playing a trick, though, because when the two children met each other, neither was a girl. And though the mothers might have dropped the idea of marriage, it seems destiny might have other plans: the lives of the two boys may be bound by a red thread.

Story1 (ch1-3) - Unmei ni Kiss
Story2 (ch4) - Graduation
Story3 (ch5) - Ordinary happiness
Afterword (extra)