Separateness is creation because everything is created from something, and the very fact that it is created from something creates a dual nature. But duality is more about opposites than it is about numbers in the creative process. Opposites are identical in nature but different in degree.
"Everything is Dual; everything has poles; everything has its pair of opposites; like and unlike are the same; opposites are identical in nature, but different in degree; extremes meet; all truths are but half-truths; all paradoxes may be reconciled." — The Kybalion.