  We explore kinetic mixing between two Abelian gauge theories that have both
electric and magnetic charges. When one of the photons becomes massive, novel
effects arise in the low-energy effective theory, including the failure of
Dirac charge quantization as particles from one sector obtain parametrically
small couplings to the photon of the other. We maintain a manifest SL(2,Z)
duality throughout our analysis, which is the diagonal subgroup of the
dualities of the two un-mixed gauge theories.
