It's commonly understood that Parmenides denied the reality of change. That the world we perceive is an illusion. But is Parmenides better understood as the SEP indicates:
Both Plato and Aristotle understood Parmenides as perhaps the first to have developed the idea that apprehension of what is unchanging is of a different order epistemologically than apprehension of things subject to change.
That is the changing every day world is real, that there is an unchanging world that is also real. That both are dependent on each other. That both are epistemologically real. One via reason, the other via sense. That neither is epistemologically or ontologically prior to the other.
On this reading Parmenides has isolated and distilled the idea of an unchanging aspect of the world from our immediate apprehension of a changing world universalised by Heraclitus.
From an early time Heraclitus was seen as the representative of universal flux in contrast to Parmenides, the representative of universal stasis. Cratylus brought Heraclitus' philosophy to Athens, where Plato heard it. Plato seems to have used Heraclitus' theory (as interpreted by Cratylus) as a model for the sensible world, as he used Parmenides’ theory for the intelligible world.
It appears to me that both Parmenides & Heraclitus were both aware of a sensible & intelligible world, and that each emphasised one side of this. Is this a form of Dualism? The SEP entry on Dualism is focused on the Philosophy of Mind, but in passing it does mention that there are other kinds of Dualism.