My boiled down take on Rudolf Steiner's explanation is that Lucifer represents the belief in separation and Ahriman represents the belief in materialism. And together they strengthen each other. The belief in separation supports materialism. And materialism supports the idea of separation.
Christ is the third principle which is unity. That's basically nonduality! Lucifer caused humanity to fall (separated from oneness). And Ahriman made humanity build the Tower of Babel, meaning developing on its own. That has been a necessary development and the return of Christ is the unification back into oneness, as a developed civilization instead of the undeveloped state of oneness humanity had in the garden of Eden.
Christ is the third principle which is unity. That's basically nonduality! Lucifer caused humanity to fall (separated from oneness). And Ahriman made humanity build the Tower of Babel, meaning developing on its own. That has been a necessary development and the return of Christ is the unification back into oneness, as a developed civilization instead of the undeveloped state of oneness humanity had in the garden of Eden.