Tight confinement of photons and phonons in nano-scale optical waveguides or cavities has revealed a rich landscape of optoacoustic phenomena. We have continued to study the "dual-nanoweb" fibre, first proposed at MPL [Butsch (2012)]. It consists of two closely-spaced ultra-thin nanowebs, suspended in a capillary, that act as optically coupled planar waveguides, supporting eigenmodes with symmetric and anti-symmetric transverse field distributions. Optical gradient forces deflect the mechanically compliant nanowebs and cause an increase in modal refractive index with launched optical power [Butsch (2012a)]. This optomechanical nonlinearity is strongly enhanced when the fundamental flexural resonance of the nanowebs is driven. Simulations show that the frequencies of upper and lower nanoweb are slightly different due to structural asymmetry.