Heaven = "Logical AND" e.g.
"I have everything, AND so do you"
"God is made of love, AND so am I"
Ego World = "Logical Exclusive OR"
In ego, no two objects share anything. You can say something like:
(ObjectA but NOT ObjectB) OR (ObjectB but NOT ObjectA)
The ego combines negation "NOT" in order to exclude, and combines it with selection between two things "OR", which doesn't allow for "BOTH", producing an "exclusive OR" - a fundamental state of inequality and difference.
When one thing is one way, it MUST be different to another thing, therefore the other thing is not the same. e.g.
"An apple is not an orange"
"Its either an apple or an orange but not both"
"I have the apple, therefore you do not"
This entire universe and all the "laws" within it are based on this ego logic of "exclusion".
The entirety of Heaven is based on the simple logic of "inclusion", which is the logical "AND" operation.
ACIM asks us to learn to "think like God": "To think like God is to share His certainty of what you are. And to CREATE like Him is to share the perfect love He shares with YOU. To this the Holy Spirit leads you, that your joy may be complete because the Kingdom of God is whole."
Wholeness, and sharing, entail "AND". We BOTH have everything. Me AND you. When it stops being AND and goes into "either or but not both", you're going into ego.
It should be obvious that Heaven is all about "inclusiveness" and Earth/this universe we made is all about "exclusiveness". That is why "Heaven and Earth are opposites in every way."