  Here, extending our previous work on this topic, we derive a dynamic
closed-form dispersion relation for a rigorous analysis of guided wave
propagation along coupled parallel linear arrays of plasmonic nanoparticles,
operating as optical 'two-line' waveguides. Compared to linear arrays of
nanoparticles, our results suggest that these waveguides may support longer
propagation lengths and more confined beams, operating analogously to
transmission-line segments at lower frequencies. Our formulation fully takes
into account the whole dynamic interaction among the infinite number of
nanoparticles composing the parallel arrays, considering also realistic
presence of losses and the frequency dispersion of the involved plasmonic
materials, providing further physical insights into the guidance properties
that characterize this geometry.
