Ok so I sort of remember this one yaoi manga, pretty sure it was a one shot, about these two friends being stuck at the hip. There was an incident when they were kids about Guy B saving Guy A I think, I'm a little fuzzy on that part. I'm pretty sure Guy B fell and hit his head.
Guy A wants to branch out. Guy B is like a puppy following his every command. Until Guy A realized that he can't escape because the fall that Guy B took when he was a kid messed with hus head and made him a yandere. I meant yandere goddammit.