The Collaborative International Dictionary
Ditheism \Di"the*ism\, n. [Pref. di- + theism: cf. F. dith['e]isme.] The doctrine of those who maintain the existence of two gods or of two original principles (as in Manicheism), one good and one evil; dualism.
Douglas Harper's Etymology Dictionary
n. A belief in two deities, which may be in conflict with each other.