  Theories with both electric and magnetic charges ("mutually non-local"
theories) have several major obstacles to calculating scattering amplitudes.
Even when the interaction arises through the kinetic mixing of two, otherwise
independent, U(1)'s, so that all low-energy interactions are perturbative,
difficulties remain: using a self-dual, local formalism leads to spurious poles
at any finite order in perturbation theory. Correct calculations must show how
the spurious poles cancel in observable scattering amplitudes. Consistency
requires that one type of charge is confined as a result of one of the U(1)'s
being broken. Here we show how the constraints of confinement and parity
conservation on observable processes manages to cancel the spurious poles in
scattering and pair production amplitudes, paving the way for systematic
studies of the experimental signatures of "dark" electric-magnetic processes.
Along the way we demonstrate some novel effects in electric-magnetic
interactions, including that the amplitude for single photon production of
magnetic particles by electric particles vanishes.
