Nanoscale thin films are essential components in modern electronics. As device power density continues to increase with miniaturization and integration, precise characterization of heat transfer in these films has become a critical issue in thermal design. However, thermal property measurements of nanoscale thin films remain challenging, particularly for in-plane thermal conductivity. Consequently, a broadly applicable method for characterizing in-plane thermal transport in nanoscale thin films is required. In this work, we propose a multilayer stacking method for measuring the in-plane thermal conductivity of nanoscale thin films using frequency-domain thermoreflectance (FDTR). FDTR is a well-established technique for nanoscale thermal characterization, known for its relatively simple setup and high accuracy. In conventional FDTR, sensitivity to the in-plane thermal conductivity of a nanoscale thin film is intrinsically low due to its extremely small thickness, making reliable determination difficult. To overcome this limitation, we propose a multilayer structure in which the target thin film and an interlayer are alternately deposited, making the measurement sensitive to in-plane heat flow within the film while preserving their nanoscale thicknesses. By analyzing the FDTR response with a fitting procedure that constrains identical parameters within the stack to shared values, sensitivity to the in-plane thermal conductivity of the target film is significantly enhanced, enabling precise determination. The effectiveness and applicability of the proposed method are evaluated through theoretical calculations based on a heat-diffusion model. The results demonstrate that this approach enables precise measurement of the in-plane thermal conductivity for a wide range of nanoscale thin films commonly used in electronic devices, with thicknesses of 1-100 nm and thermal conductivities of 1-100 W/(m·K). This method provides a precise, broadly applicable framework for characterizing in-plane thermal transport in nanoscale thin films, contributing to a deeper understanding of nanoscale heat transport and to improved thermal design of advanced electronic devices.