No, that’s not how twins work…
Identical twins occur when a fertilised egg splits in half. In the case of parasitic twins, when the embryos begin to develop, they DO NOT fully separate, with one being more dominant than the other in its growth (taking the majority of the nutrients to grow), hence the other being called the “parasite” and it it wholly depends on the dominant twin and incomplete in formation.
In the case of Kang Mengru, her “parasitic” twin lied within her persons, otherwise known as “foetus in foetu”, and it usually ensues with the parasite becoming into a tumour-like growth which needs to be removed as it poses a threat to the otherwise “healthy” twin.