Tunneling electron transfer rates are calculated in a pair of coupled bistable oscillators within thhe Piepho-Krausz-Schatz (PKS) model. Both elastic and vibronic couplings are considered and found to give very close results. Various two-electron tunneling transfer modes are characterized. In particular the ground tunneling frequency can be strongly lowered with respect to the uncoupled oscillator pair. This treatment is applied quantitatively to the mixed-valence biferrocenium cation. © 1994.