The integration of the cryosphere into a modular digital twin of the Earth is a central goal of the EU project TerraDT. Existing Earth system models (ESMs) typically only include ocean, land, and atmosphere components and additional physical effects have to be incorporated via coupling to avoid starting over from scratch. Beyond Earth system modelling, code coupling also plays an important role in many engineering applications and, more generally, in multiphysics problems. A well-known example is fluid-structure interaction, where a strong coupling between fluid dynamics and structural mechanics is essential. In this talk, I focus on the partitioned approach to model coupling, in which independent simulation codes are coupled in a minimally invasive manner. While this approach offers flexibility and reduces development effort through code reuse, it also comes with a broad range of challenges: specialized coupling schemes are needed to ensure stability and convergence, efficient communication infrastructure is required for HPC systems, and data must be mapped consistently between different meshes. Coupling libraries provide standardized solutions to address these challenges. As with any software, their success strongly depends on interface design and the availability of appropriate features, which in turn reflect the needs of the target user community. Consequently, coupling libraries are often tailored to the specific needs of a target group. To illustrate this, I discuss and compare the implementation of key features in two coupling libraries: YAC, a domain-specific library for ESMs, and preCICE, a multiphysics coupling library with a broad user community.