Sandhya S. mentions both of these concepts in her article Sunlight, Healing, & Energy Flow.
Santana Dharma
Santana Dharma the term can be synonymous with Hinduism as a collective philosophical thought and belief system since the origin of the term is in from India. BUT it is not limited to a religious institution or body. It was used as a socio-political unifier in the past since there are many religious sects within the Hindu culture. In this definition, it means that any being who recognizes that "thou art that." Because of this knowingness or recognition, they choose to follow a set of ethical guidelines and spiritual practices that allow one to reach a state of spiritual liberation (moksha). This includes the practice of yoga as a vital part of the process as it brings a being towards self-realization allowing for union with their spiritual nature. Another understanding of this or approach to this term is that sanatana-dharma, the self/Self (atman) is to perform service based on universal laws that are transcendent and beyond a temporary belief system.
Tat tvam asi: Thou art that.
Thou art that (Tat tvam asi) does not differentiate or particularize any one being. Instead, it describes the relationship between the individual identity that is consciousness exper-ienced through a particular nervous system as the ineffable - the absolute - the supreme consciousness or all that is and ever was. Chandogya Upanishad, Chapter 6 (c. 600 BCE) as the teacher Uddalaka Aruni instructs his son in the nature of Brahman, the supreme reality.