The effect of quadratic friction on tidal currents consisting of equal M2 and S2 constituents is considered. It is shown by two separate arguments-one based on an energy balance, the other on harmonic analysis of the frictional force-that, for rectilinear and parallel currents, the drag coefficient applicable to either constituent considered in isolation should be 1.70 times greater than that applied to the two constituents propagating together. The results of a numerical model of the tides in Gulf St Vincent, Australia (where equal M2 and S2 tides occur) are consistent with this prediction. A general result of this work is that the drag coefficients predicted by harmonic analysis of the friction force give the correct rate of energy dissipation for any number of tidal constituents, equal or not.