There are two gods. The "Good Spirit" is Ahura Mazda and the "Evil Spirit" is Angra Mainyu. Their relationship to one another is very similar to Yin and Yang.
The portions of the Avesta of Zarathustra which survived Islam's attempt to wipe from the face of the Earth are quite interesting reading. Also - though often considered apocrypha - the Desatir is very beautiful. I suggest them both.
The idea of Zarathustrian thought being Monotheistic is actually a very common error.
There are two gods. The "Good Spirit" is Ahura Mazda and the "Evil Spirit" is Angra Mainyu. Their relationship to one another is very similar to Yin and Yang.
You are simply playing word games.
The point is Zarathustrians only worship one god, so they are monotheists.
You could just as easily say that there are two "gods" in Christianity: "God" and the Devil. Or three "good gods": Father, Son and Holy Ghost. Or many, many gods if you define angels and saints as "gods", and why not, since they manifest most of the attributes of gods?
All monotheists did (immitating Zoroaster/Zarathustra) was to condemn all the old gods as "devils" and to devote all their devotion to a new "good" god supposedly in charge of everything.
But even the strictest monotheism is dualist in admitting a power of evil, either equal or inferior in power to that of good, but god-like none the less. And the good and evil gods have lots of helpers, angels and demons, so what we have here is still the same old polytheism under a new name, with all the credit going to the "good god" who is supposed to be running the show.
Conversely, all polytheists admit to a greater unifying power that could be called "god" and is therefore partly monotheistic in nature. Polytheists tend not to worship "jealous" gods, naturally. Worship a jealous god, and you end up being a monotheist.
Christianity itself is simply a renamed/repurposed Zoroastrianism, by way of Mithraism and messianic Judaism (such as the Essenes and other no longer extant Jewish groups). Much of Christian ritual and belief is virtually identical to much older Mithraism and Zoroastrianism, and its eschatology and implicit dualism, though coming to Christianity through the half-way house of messianic Judaism, is straight from Persia, and is unadulterated Zorastrianism in its core assumptions.