The whole argument hinges on the idea that humans have souls in the first place. No adequate theory pertaining to the existence of human souls has overcome the challenge of how a non-physical soul can interact with a physical body. Why, for example, when my "soul" wants to move my arm, does my arm move, and not your arm, or the cat's arm? The laws of physics do not permit the interaction between a physical object and a non-physical one.
Without proving the existence of the soul, spiritualists cannot prove the theory of love as two souls coming together.
Even if we can accept the existence of a soul, this view of love is constructed around monogamous, binary views of love and sex. It does not extend the existence of love to those in polyamorous relationships. This restrictive and binary view of love is outdated in the modern world where polyamory is increasing and a more progressive attitude towards love is emerging.