Question for clarity?. The current aerator outlet manifold does not have ball valves??? Ball valves should be used on dual or multiple outlets on an aerator manifold. One ball valve per outlet. Ball valves are needed to regulate flow of air especially if diffusers are at different depths. If it were my manifold, I would install some sort of ball valve on each outlet. (Gas line ball valves are a good option.) Then in your case you can send all the air to the one diffuser and allow a tiny bit of air to escape out the other outlet but this may not be necessary. Releasing air or plugging the unused outlet depends on type or brand of diffuser and the amount of air volume produced by the compressor. Diffusers are normally rated to accept a certain or maximum amount of air to operate efficiently and not damage the diffuser membrane. Also when pushing too much air into the diffuser beyond what the diffuser is rated to have,,, this opens the diffuser slits too wide and causes the diffuser to not efficiently utilize the air for optimum bubble production and optimum water uplifting movement.