Interesting but no, this is more like an extreme more perfect union of the two siblings. See before the two characters were born, at about the eight to sixteen cell stage of development, the character's zygotes merged.

*exlaimr* WARNING: graphic sciencey type stuff coming up! since their genes came from two separate eggs and were fertilized by separate sperm, their genomes are different. One twin was going to be a boy and the surviving twin absorbed her unborn sibling. As a result, only one baby was born and assigned female at birth.

The brother—at an unnoticeable level—had enough of his brain and endocrine system left that he technically shares his sister's body and is developing along side her. This is scientifically known as the chimera effect. It usually doesn't involve a separate consciousness but since this is fiction, I figure why not make it more interesting?

Fun fact for people who don't know: there have been documented cases of the chimera effect in birds, reptiles and in humans. It's led to some really freaky drama.(Including trouble with paternity tests, income taxes problems and medical drama.)