We propose an alternative way to enhance optical nonlinearity from nanocomposites containing nonlinear bimetallic nanostructures. The optical nonlinearity enhancement are due to two aspects: firstly, the intrinsic large nonlinearity of metallic material in core and shell; secondly, the coupling of dipole modes gives rise to symmetric and antisymmetric resonance, which strengthen the local fields in the core and shell. By fine adjustment of the core-shell size ratio, it is possible to achieve maximal enhancement of optical nonlinearity from bimetallic coated nanocomposites.