The formalism of quantum electrodynamics for treating the interelectronic-interaction correction of first order
in 1/Z to the two-electron part of the nuclear recoil effect on binding energies in atoms and ions is developed.
The nonperturbative (in αZ) calculations of the corresponding contribution to the energies of the 1s2 state in
He-like and the 1s2 2s and 1s2 2p1/2 states in Li-like ions are performed in the range Z = 5–100. The behavior of
the two-electron part of the nuclear recoil effect beyond the lowest-order relativistic approximation as a function
of Z is studied.