In ACIM Jesus describes that revelation is direct communication between God and one of his creations.
It also describes revelation as intensely personal.
It also describes revelation as being knowledge which cannot be directly shared, but that God wants us to bring or express the truth of the knowledge to OTHERS.
Now, we can do some pretty simple logical reasoning here.
1) God can only communicate directly with beings that He directly created, with no go-between.
2) He must communicate immediately and directly to whatever He created.
3) Whatever He created must be in Heaven and must be real.
4) Revelation is not given to the ego, it's given to God's creation. The ego is not part of His Creation and He does not communicate with it.
5) Since revelation is "intensely personal" and it's given to a real creation in heaven, and yet cannot be directly shared with others, there MUST be "personal selves" in heaven.
6) Since God wants the knowledge of the revelation to be shared with OTHERS, there must BE others in heaven.
7) Therefore God must have multiple sons who He DIRECTLY CREATED IN HEAVEN, which are real and permanent. Which also means, the idea of a "one son" who somehow encompasses the sonship or is the ONLY Son, is incorrect. The One Son is the SHARED EXPERIENCE of all DIRECTLY CREATED Sons, not a go-between or umbrella.
8) Given that God can communicate directly and fully to ONE of His creations, in an experience of transmission which goes ONLY to that one creation and not to other creations, and cannot be shared BY other creations at the same time, this MUST mean that there is some kind of "separation" or division IN heaven distinguishing one self from another, preventing the revelation from being shared.
9) Therefore Heaven cannot be a state of "no separations" or "no individuality" or "no free will".
10) Therefore ideas of non-duality and "oneness" meaning "only one thing" with "no separate parts" is a very poor fit for the truth of A Course in Miracles.
"God, who encompasses all being, nevertheless created separate beings who have everything individually, and are willing to share it to increase their joy." - ACIM