The ending is definitely rushed and i feel the hyung should have immediately sought therapy before everything became all happy dandy to make sure his mental health is alright and that the mc should have had the time to come to terms with the selfishness of his behaviour and make amends. A running theme in this story is the kindness of the mc which the hyung was attracted to and this caused him to suppress aspects of his personality he deemed unworthy of such kindness and the feelings he had for the mc hence he drew a line between them causing the split personality.
The mc's decision to not tell his hyung immediately and get him a good psychiatrist for the sake of his own carnal pleasure is a very selfish act, and things could have gone really wrong, what if fusion wasn't a possibility and what if the hyung couldn't come to terms with himself/feelings and decided to take drastic measures because of his confusion with memories and maybe decided to cut off all contact or physically harm himself.
While i understand were both of them are coming from, it is quite concerning that it is not made clear the possible consequences of his actions and when you consider that the people around the mc have a good grasp on human psychology it makes it inconsistent and out of place, not to mention before liking hyung romantically, the mc saw him as a very important person that was practically family, not informing him of his mental health issues makes me question that initial familial love.
All in all, it wasn't bad, the story was cute and the sex scenes were well drawn. The second couple are quite interesting and the ending was rushed but alright.