If the second manifold/ coil assembly in the double-unit (V7502) system is not being used, it is NOT necessary to plug either fitting on top of the unused manifold, as it is a closed loop that will not leak water from the cooling reservoir tank.
However, if only one autoclave is connected to the system, it is important to plug the unused fitting on the drain adapter assembly with the provided plug to prevent any possible splattering/ leakage at the drain connection.