So far as I can determine, it was the late Ray Buckland (1934-2017) who first coined the ghastly word duotheism to describe Wicca's two-deity, God-Goddess system.
Well, Uncle Ray accomplished many things in his 83 years, and all New World pagans owe him a deep debt of gratitude.
But a poet, he wasn't.
The term duotheism first saw print in the introduction to his 1974 The Tree: A Complete Book of Saxon Witchcraft. In the absence of any earlier citations, my guess would be that it arose out of the theological ruminations that went into the making of Seax Wicca.
Alas, this amateurish botch of a coinage still sees light every now and then. It still makes me cringe every time.
In fact, the coinage was unnecessary. In the language of religious scholarship, a two-deity system is known as a bitheism. To this poet's ear, at least, this is a much less painful option.
Aesthetically speaking, that is.
As for the theology of the matter, well, don't look at me: I'm not Bi.
I'm Poly.