Duotheism is the in belief two deities, most commonly in a single god and a single goddess, although it can also be used to regard a mutual power between two deities. It is sometimes explained with the phrase, "All the gods are one god and all the goddesses are one goddess," purportedly from the Eleusinian Mysteries.

A duotheistic understanding of the divine is often found in popular books about Wicca and neo-Paganism, which often refer to "The Goddess" and her consort, "The God". Nowadays more and more Wiccans are taking a polytheistic stance, as it becomes difficult to sustain syncretism on such a massive scale.

(Lifted from Wikipedia as I wrote the original entry on duotheism as far as I can remember - Yvonne Aburrow)