  It has long been known that in the absence of electric charges and currents,
Maxwell's electromagnetism in 4 dimensional vacuum Minkowski space-time is
invariant under SO(2) dual transformations that mix its electric and magnetic
fields. Extending this symmetry to include the coupling to electrically charged
matter, requires a dual coupling to magnetically charged matter as well,
leading to Maxwell equations for SO(2) dual electrodynamics. Based on a doubled
ensemble of SO(2) dual 4-vector gauge potentials which does away with the need
of Dirac string singularities for magnetic monopoles, a local Lagrangian action
principle for SO(2) dual electromagnetism is known, which manifestly displays
all the required space-time and internal symmetries, and reduces to the
experimentally well established Maxwell electrodynamics in the absence of
magnetic charges and currents. Applying the same considerations for the matter
action of electrically and magnetically charged point particles, a unique SO(2)
dual generalised Lorentz force is identified for SO(2) dual electrodynamics,
truly different from the usual SO(2) dual invariant choice motivated by
simplicity, but yet made arbitrarily and which does not derive from some action
principle. This generalised Lorentz force involves a single real and new
coupling constant of unknown value, without the requirement of a
Dirac-Schwinger-Zwanziger quantisation condition for electric and magnetic
charges of dyons. A physical consequence for SO(2) dual electrodynamics of this
coupling constant if nonvanishing, is to open a channel, or portal between the
otherwise mutually totally ``dark'' sectors of electric and magnetic charges
for electromagnetic interactions.
