Potential energy surfaces are generated by ab initio calculations for the transfer of two protons in the pentameric chains (H2O ·· H2O ·· H2OH+ ·· OH2 ·· OH2) and (H3N ·· H3N ·· H3NH+ NH3 ·· NH3). These surfaces are then fit by the sum of (a) two functions, each of which reproduces the transfer potential of a single proton plus (b) a function that models the coupling between the two protons. Several candidates are tested for the latter coupling function. A simple linear dependence upon the motion of the two protons reproduces the full two–dimensional surface to an accuracy of some 1 kcal/mol. The popular quadratic function provides a poorer approximation as also do exponential and Coulombic-type functions.