The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]