I use two diffusers in my aquarium, one for my CO2 and another instead of an air stone for my air pump. I am thinking about using a couple of check-valves and a splitter to hook both my CO2 and air pump to 1 diffuser. I'm wondering if anyone else has done this, and what sort of results they got? Any input would be helpful even if you have no experience with this. :]
In my opinion, connecting a CO2 source to an air pump using check valves, and then going to a single diffuser would be ineffective.
With CO2 you want extremely small bubbles that will rise slowly on their own to the top of the tank. This gives the maximum amount of time for the co2 to get absorbed into the water.
With air you are generally trying to get water movement, so you want much larger bubbles. This moves more water.
You also have an additional problem. unless the pressure from the CO2 and the air pump is equal, the higher pressure source will keep the other source closed off. This is because the pressure on the check valve to keep it closed is greater than the pressure trying to open the check valve.
Is your goal to have the air come on when the co2 is off? If so, and your co2 is controlled by a solenoid just put your pump on a timer to come on when co2 is off. The check valves on both sides should keep the flow going in the right direction.