Composite sandwich panels can support high loads while maintaining a low average weight due to their geometric design. However, using standardized finite element methods for calculations often leads to large models and extended computation times due to complex geometries. This contribution investigates the mechanical properties of these panels through a multiscale approach that employs structure-structure homogenization to simplify the intricate mesostructure, offering a more accurate representation while significantly reducing computation time. By applying a multiscale approach, the problem is divided into two scales: the macroscopic scale describes the overall geometry and loading conditions, while the mesoscopic scale characterizes morphology and averaged physical properties. We build upon existing frameworks for carbonreinforced concrete shell structures, discretizing the structural element at the macroscopic scale with standard shell elements while applying a Representative Volume Element (RVE) at the mesoscopic scale. Specifically, an RVE is assigned to each integration point on the macroscopic element level. Following this, once the macroscopic shell strains are applied to the RVE, we proceed to solve the mesoscopic boundary value problem, ultimately yielding the homogenized shell stress resultants and the shell material tangent operator. This approach is expanded by a structure-structure homogenization using Reissner-Mindlin shells within the RVE framework, distinguishing it from traditional solid 3D element models. To connect these scales, we apply the Hill-Mandel condition, ensuring energetic equivalence through appropriate boundary conditions for the RVE. In this work, periodic boundary conditions prescribing the displacements and rotations are applied. This approach ensures proper deformation modes and prevents length dependency of the membrane and bending entries of the material tangent operator. Our findings are compared to methods that use periodic boundary conditions for the displacements and transition elements to prescribe the rotations, as well as to methods employing displacement boundary conditions.