I believe it was an infinite flow/horror genre BL. In one arc I believe the ML split into two people (brother and sister) who were both protecting MC? I think one of them (the sister?) sustained wounds/died protecting him as well, and that the ML wore glasses. For that arc, I believe it was a gothic/European theme where they were staying in a castle. Thanks for anyone who might know what novel this was!
May not be the right one but Supernatural Movie Actor App has a main with a split personality (which is the ML) and a side character has a split personality (brother and sister). It’s also an unlimited flow horror novel