Separation is the idea that there is an opposite to love.
It has given rise to a world where both love and hate seem to coexist, where there is both joy and sadness, and where there is both Life and death.
The "duality" of the world is therefore a picture of the belief that Life and death both coexist, that death has been added to the realm of God's existence, and that it's possible for opposites to occur simultaneously.
There seems to be duality in the world because the ego believes in conflicting goals and opposite possibilities. We seem to now experience a world of both life and death, happiness and sadness, love and hate, certainty and fear.
We say that the hate and the unhappiness and the death and the fear are the problem that the ego created. But moreso it's the belief that these things can exist and that they are separate from the good stuff - love/light/joy/peace - that creates this illusion of a split.
The world is an idea of the introduction of an opposite to Life, an opposite to love and an opposite to happiness. It therefore has to manifest forms which depict both of those opposites. To have one without the other would seem to be impossible. Where there is life there must be death.
But in truth, real life does not require death in order to exist. Death on the other hand requires life in order to define itself as its opposite. Death is not a true opposite to life. Life is eternal and death is nothing.