Union in yoga. Being one with God in Christianity. Enlightenment in Buddhism and other traditions.
I haven't read the book, but this masculine/feminine split is present in every 'spiritual' model I've worked with so far. Each of those systems culminates in some form of union, but Buddhism seems to be quite different in that respect; Buddhism aims towards nibbana, which even the word "union" can't describe since it's devoid of any such opposites that would allow a "union" to take place.
The idea is a departure from the basic spiritual tenet of absolute oneness, to a theory of basic masculine and feminine manifestations of reality. The idea is that the universe, Absolute Reality, God, whatever you want to call it, exists but is unconscious of itself, and the purpose of the Absolute, is the design of intelligence, consciousness, self awareness, and that purpose of that intelligence, is to realize the absolute.
It's not really such a big departure as you may think, the idea of "God" (or whatever) being unconscious of itself is common in a lot of Eastern traditions; the Sufi's have a nice way of describing it, they say that "God" isn't unconscious, just really, really drunk.

Also, what does this theory imply about the fundamental concept of non-duality??
Non-duality is just one phase of this, don't hang onto it too much 'cause the rabbit hole goes much, much deeper and gets more and more fun by the day!