The maximum power of a combined endo-reversible engine and its corresponding overall thermal efficiency are studied subject to the design parameters considered. The maximum power of a combined endo-reversible heat engine and its corresponding thermal efficiency at this power are derived both analytically and numerically and assessed according to the extreme temperature ratio and the thermal conductance which is the most important parameter used in the heat engines. These derived equations can be used practically for the preliminary design of combined heat engines since they contain design parameters. The analytically and numerically derived results are compared with each other and also compared to previous studies on this topic. As a result of these comparisons, some differences are observed between this study and previous ones in terms of thermal efficiencies at maximum power and the upper limit ratios of maximum power when the maximum power of a combined endo-reversible engine and a single engine are compared. The reasons for these differences are discussed.