Phase change materials (PCMs) are commonly used energy-storage media in thermal storage devices. Shell-and-tube heat storage units represent one of the most widely adopted configurations, yet most existing studies focus on systems with a single temperature boundary, while investigations on heat-storage performance under multiple temperature boundaries remain limited. In this work, a double-tube shell-and-tube latent heat storage model featuring dual temperature boundaries was developed, and numerical simulations of the heat-storage process were conducted using the COMSOL Multiphysics software.The results show that, under a constant temperature difference between the upper and lower heating tubes, altering their vertical arrangement can significantly enhance the melting rate of the PCM and improve thermal-storage efficiency. The temperature difference between the heating tubes has a pronounced effect on the complete melting time, although the total stored energy varies only slightly under the same temperature difference. Moreover, the influence of tube spacing on melting time differs depending on the tube-arrangement conditions.A comparative analysis between natural-convection and pure-conduction models was also carried out. In the pure-conduction model, the melting-time difference between the two heating-tube arrangements is relatively small; however, in the natural-convection model, the configuration with the upper tube at a lower temperature and the lower tube at a higher temperature exhibits a more significant convection-enhancement effect. As tube spacing increases, the natural-convection enhancement coefficient λ for the “upper-low / lower-high” temperature arrangement is clearly superior to that of the “upper-high / lower-low” arrangement. This study offers new insights for the design and optimization of phase-change heat storage units under multi-temperature-boundary conditions and provides theoretical support for identifying melting mechanisms.