When Tārā assumes the qualities of Śiva, Kālī gets to be the counterpart - Śakti and when Kālī assumes the part of Śiva, Tārā will become Śakti. The clever sādhaka as a result, should under no circumstances understand any difference between them and therefore gets liberated, by worshipping both https://www.youtube.com/watch?v=GMMWwN9ho2U&t=6s