Eiko, I'm just going to out on a limb here, mainly because we all want to know the answer to your riddle.

It's kid's heart. It's with her for her entire life, and her heart (Or morales, ethics, [Insert Synonym]) is what drives her to do the things she does.

Due to the fact that Kid is Schalas clone, "Two of me" fits the heart, because they both have the same heart. The 'Heart' of both Schala and Kid (Think of it as one entity) makes life for Kid much easier than for Schala, who is mildly conscious of what she has become.

That is far out.