Coreless electric motors are gaining interest from the community as they have a larger power-to-weight ratio and a larger efficiency. The absence of iron slots leaves the winding space unobstructed. This enables the geometry of the coils to be skewed to overlap with their neighbors. Such arrangement provides better motor constant. This work proposes to establish some groundwork for the topology optimization of overlapping winding in multi-phase coreless motors. A 2.5D formulation of the density-based approach is proposed to enable the designed coils to span over two layers and spatially overlap with their neighbors, while keeping a limited computational cost. A novel interpolation scheme is also proposed to account for multi-phase winding while only designing one phase.