‘Not-Two’: Àdvaitham, a term that long predates Shūnyam. Yājñavalkya defined it as simply: ‘ Neither before nor after; Neither inside, nor outside’.
It has lots of layers to it and you won’t really notice them until you slip on one. But the three most relevant can be readily listed.
The first is the assumption of the ‘Independent and Separate ‘Self’. The ‘Two’ of the ‘Subject-Object Divide’. [‘Self and World’; ‘God and Man’, and so on.]
Secondly, the open-ended: ‘Not’.
Thirdly, our reflexive tendency to abstract in Sign and Symbol [‘Doubles that Refer’] and hence make our World amenable to Logic and Language.
‘Not-Two’ is a statement of Truth, not an appellation, not a name for an ‘Object’ [concept, process, state, sentiment, anything you can objectify]. And the confounding of ‘Not-Two’ as a conventional reference, a name, is pervasive in the historical literature.
‘Not-Two’ actively locks in the Subject in a verbal hog-tie. You may not not say a word [or write a Post] about it. Except to call it ‘Not-Two’.
While ‘Not-Two’ is very helpful aid, a GPS addition to navigate the ‘Backward Step’, you always and only orient to Shūnyam, never to ‘Not-Two. Hence also, as shunyam.org