I think you would have a hard time regulating how much came from each source because of the difference in head pressures. Even with the control valves most of the water would come from the main tank because head pressure was working to its advantage. I can see what you are trying to do, have a closed loop without buying a seperate pump. I just don't know if it would be practical. I'd be afraid you would end up with water on the floor if one of the valves got blocked.