Adsorption isotherms of dimers on random heterogeneous surfaces, based on the lattice gas model, are calculated by the grand canonical ensemble Monte Carlo simulation method. Random heterogeneous surfaces are presumed to consist of two active sites with different energies epsilon(a) and epsilon(b) which are randomly distributed on a simple square lattice, on which dimer molecules composed of the identical segments occupy two adjacent sites without lateral interactions. Calculated adsorption isotherms are used to test the applicability of two versions of the multisite occupancy model originally proposed by Nitta et al. (1984); one is characterized by the quasi-chemical approximation for segment-site pairs and another is by molecule-bond pairs. The two model equations are found to be good only when the difference in r(0) = exp{(epsilon(b) - epsilon(a))/kT} is smaller than 10. For r(b) values greater than 50, some discrepancies are observed in such a way that the first model overestimates the energetically more stable states while the second model underestimates them.