Ok so you have 2 valves one on a flow in line and one on a flow out of the tank
-the level will be controlled by the outlet valve
-flow controlled by inlet valve
when level gets out of whack you want the level loop to essentially take control of the flow loop and increase or decrease substantially based on the direction of your error.
I am a bit confused because if you have a controlled constant flow in and are controlling level on the outlet, "flow out", when in equilibrium nothing should cause you to have to leave equilibrium. or are there more than one inlet/outlet on the tank
If the process is what I think it is you shouldn't need anything but the level PID controller to keep you level at setpoint or is there a reason your level controller can't compensate for the error itself by using the outlet valve, ie do you have a min and max flow out?
override or high low selector will work and what you will essential have is a pid adjusting the input and output of your process simultaneously this may affect your tuning constants and process stability.