This work investigates the simultaneous optimization of topology and interfaces in the design of large-scale assembled structures for additive manufacturing. Owing to limitations in build size and process stability, segmented fabrication is often required, which poses challenges to structural integrity and performance consistency after assembly. To address these issues, an explicit simultaneous optimization method is proposed, in which the topologies of substructures and their assembly interfaces are optimized within a unified framework. Departing from the conventional continuum assumption, the proposed approach decomposes a large structure into multiple substructures and simultaneously optimizes their topological configurations and interface connections, aiming to achieve optimal global mechanical performance and enhanced interface stability. The moving morphable voids (MMV) method is employed to explicitly describe structural topology, while the mechanical behavior of assembly interfaces is characterized using cohesive laws combined with shape parameterization based on coordinate perturbation. During the optimization process, topology evolution driven by MMV is combined with interface shape optimization, enabling a simultaneous design of topology and interfaces. An adaptive mesh discretization strategy is adopted to support stable and accurate numerical analysis of segmented structures with complex topologies and multiple interfaces. For stiffness optimization problems, analytical shape sensitivities are derived within a variational and adjoint framework, enabling stable gradient-based optimization. In addition to adhesive interface configurations, topology optimization of self-assembling structures inspired by traditional Chinese mortise and tenon joints is also explored. Numerical examples demonstrate that the proposed method enables effective simultaneous optimization under different interface properties and partitioning patterns, resulting in assembled structural configurations with superior mechanical performance.