Mary Boyce's scholarship on the Zoroastrian textual tradition treats this verse as the pivot of a dispute that split the religion for centuries rather than a settled point of doctrine. Read one way, the two spirits named here, the better and the worse, are independent and coeval, each choosing its own nature freely, the reading that became orthodox Zoroastrian dualism, good and evil as two real, underived principles locked in real contest, with the outcome genuinely open until the world's final restoration.
A rival reading, associated with the Zurvanite movement that flourished especially under the Sasanian empire, held that both spirits derive from a prior principle, Zurvan, time itself, making the two spirits twins born from one source rather than independent from the start, a move that answered an uncomfortable question, if God is wholly good, where does the possibility of evil's opposite spirit come from, at the cost of making the ultimate principle behind the universe morally neutral rather than good. Zurvanism lost ground after the Arab conquest ended Sasanian patronage and the priesthood that had preserved its texts, and the surviving Zoroastrian tradition today generally holds the dualist reading Boyce's sources present as the older and more textually direct one, though the dispute this verse generated shaped Zoroastrian theology through the whole of the tradition's classical period.