  We study the interaction of emitters with a composite waveguide formed from
two parallel optical nanofibers in currently unexplored regimes of experimental
importance for atomic gases or solid-state emitters. Using the exact dyadic
Green's function we comprehensively investigate the coupling efficiency and the
fiber-induced Lamb shift accounting for variations in emitter positions and
fiber configurations. This reveals coupling efficiencies and Purcell factors
that are enhanced considerably beyond those using a single fiber waveguide, and
robustness in the figures of merit. We finally investigate resonant
dipole-dipole interactions and the generation of entanglement between two
emitters mediated through the composite waveguide under excitation. We show
that the concurrence can be enhanced for two fiber systems, such that
entanglement may be present even in cases where it is zero for a single fiber.
All-fiber systems are simple in construction and benefit from a wealth of
existing telecommunications technologies, whilst enjoying strong couplings to
emitters and offering novel light-matter functionalities specific to slot
waveguides.
