This article presents an investigation of the linear optical properties of a dual-core photonic crystal fiber with a square lattice made of a multicomponent glass in a second communication window. An experimental method based on intermodal interference was used to determine the effective refractive indices of higher-order modes, with knowledge of the fundamental mode dispersion. The effective refractive indices were also determined by an FDTD-based simulation and the obtained values provided a good agreement in comparison to the experimental results. The obtained results help to clarify the nonlinear spectral transformation processes observed in the same fiber at propagation in the higher-order modes.