I was doing my philosophy homework tonight, and I realized a possible theory as to where the name for the Duality map came from.
If you think of Descartes, you might know of his duality philosophy. It states that there are two realities in the world, one is called thought and is the world of the mind...dreams, visions, the voices in your head, etc....and the other world is called the extension, and is the material world. The world of matter and physics. This theory made Descartes what is known as a Dualist, hence the philosophy.
The name might have been chosen because of your constantly changing position from dreams to reality during Marathon: Infinity. You were, living in a duality, so to speak.
Just a thought