Hm? No, it's not an alternate personality doing the NTR. It's just that the NTR guy's motive is that he's also the MC's stalker and he was reducing the competition.
Edit: The title is "My stalker got NTR(?)ed by a playboy." Romaji title is "Boku no suto-ka- ga yarichin ni katte ni netorare(?)teta hanashi." The premise aside from the twist ending is basically contained in the title.