The concept a human being having one soul does not hold good as per Bhagavad Gita. And why would that be? All souls started their cosmic sojourn when Brahman, God Almighty explodes self with a big bang.
The big bang explosion results in all pure souls atmans scattering all over cosmos at unimaginable speeds. In the melee, hurtling souls’ atmans gathered impurities similar as a rolling ball gathers moss. To cleanse self of dross impurities within starts cosmic life cycle of every soul.
Of its own no soul atman can purify itself, regain its lost original pure prime pristine primordial form. It needs a medium, a body to work out its karma, cleanse self of dross impurities within.
No sooner life supporting planets like mother earth evolved, souls immediately started manifesting life. Body after body, manifestation after manifestation the journey of soul continues on mother earth. A soul initially for the first time manifests form of amoeba (single cell formation). It further evolves into multi-cell formation, then insect life, plant life, animal life and finally human form.
If the human body consists of amoeba existing within then every single amoeba also possesses a soul atman within. So can be the case with many bacteria also.
Similar as many people can reside in a multistoried building, many souls’ atmans could independently work out their karma in a single human body.
As per Bhagavad Gita every single soul atman was independent of other. Governed by doctrine of evolution and karma atmans keep evolving until it manifests human form for the first time. For an atman it is only in human form liberation from cycle of birth and death becomes possible.