The two-body radiation-diffraction problem has been solved by both the conventional two-body analysis and a boundary matching method with damped free surface condition (Hong et al. 2010). Numerical results of drift forces and moments and hydrodynamic coefficients of two identical caissons floating side by side obtained by the two methods have been presented. Amplitudes and phases of the wave elevations in the gap at the resonance frequencies computed by the boundary matching method have been presented. It has been shown that the unrealistic scattering or radiation wave elevation due to the gap resonance can be reduced by applying the damped free surface condition on the free surface in the gap. The motion responses and drift forces of a container ship and a barge-type mobile harbor floating side by side have also been presented.