Maximum work from two finite heat reservoirs ($W_{\max }$)

With constant heat capacities $C_1,C_2$, a reversible heat engine taking two bodies to a common temperature has $T_f=T_1^{C_1/(C_1+C_2)}{T}_2^{C_2/(C_1+C_2)}$. The maximum work is $C_1{T}_1+C_2{T}_2-(C_1+C_2)T_f$. heat isolation does not prohibit transferring work to a store included in the total isolated system.