Let \(K_1 \subset H\) and \(K_2 \subset H\) be half-sided modular inclusions in a common standard subspace H. We prove that the inclusion \(K_1 \subset K_2\) holds if and only if we have an inclusion of spectral subspaces of the generators of the positive one-parameter groups associated to the half-sided modular inclusions \(K_1 \subset H\) and \(K_2 \subset H\) . From this we give a characterization of this situation in terms of (operator-valued) symmetric inner functions. We illustrate these characterizations with some examples of non-trivial phenomena occurring in this setting.