The earliest expansion of ‘Not-Two’ in the literature is as: ‘One without a Second’. ‘The One’ [Sanskrit: Ekam Sat] can be found in the history of every literate tradition.
The notion of ‘The One’ is universal and grades and variants of it can be found in every literate tradition from Plotinus to that of that of the Abrahamic Faiths which gets conceptualized and reified into a later ‘Monotheism'[See the Posts].
‘God is ‘Namelessness’, wrote Meister Eckhart, the pioneering Christian Mystic who taught Aristotle at the Sorbonne. ‘The eye through which I see God is the same eye through which God sees me; my eye and God’s eye are one’. [He was promptly arraigned by the Vatican].
The Neo-Platonic ‘One’ of Plotinus [sourced in Plato’s Parmenides] was the dominant European Mystical Tradition for a millennia. It influenced everyone from Thomas Aquinas to Meister Eckhart.
Reality, wrote Plotinus was: ‘A Nameless Unity, indescribable, undefinable.. never known measure, stands outside number..is under no limit of any kind..is Everything and Nothing..’.
Perhaps no other Mystic influenced all three of the Abrahamic faiths as comprehensively as Plotinus [203-270]. He famously joined Gordian’s march on Persia in order to find his way to India. The campaign failed and he returned to Antioch.
In 3rd Century Bharath, the understanding of Shūnyam was still not entirely dead. It would have been a worthwhile visit.
There is a long history of The ‘One’ being appropriated especially in the religious literature, its intuition being interpreted as a variant of Deux Factus Sum. The finer label ‘Not-Two‘ might save you from that hole but there is a copious literature maintaining the same claim for it as well. Stay alert.