We analyze the Mössbauer quadrupole data of several biferrocenium salts, using a recent two-orbital extension of a previous cooperative vibronic Piepho-Krausz-Schatz (PKS) model. A discontinuous (continuous) localized-delocalized transition is described in terms of an order-disorder transition of first (second) order. The first-order results from a crossover of vibronic states based on the two orbitals. The thermal variations of the specific heat are calculated and compared to the available experimental data. © 1994 The American Physical Society.