You dont want to put valves on the overflow. You put a valve on the line FROM the pump and up to the tank. Flow down to the sump will be based on the flow going up into the tank. Think of it this way... if your tank is full to the very top and you pour five gallons of water into it, you will have 5 gallons spill out....not 10.
By using the two drains from the overflow, all it will do is allow a higher flow rate, and as mentioned, add a little security in case on clogs, but that's pretty rare. I normally put sponges in the tank side of the overflow to help quiten it down. It also blocks any large debris from getting through. Most importantly though, it helps stop bubbles from getting up into the U pipe. As air builds in the top of the pipe, it reduces flow rate, and can lead to overflowing the tank. This is why some people will put an aqualifter on them at the top to suck out any air that accumulates.
BTW, I have a dual box, with a hose for each. It needs a good cleaning, but is in good operational condition. I could sell it for $55.