The guy I got my tank from had a single drain with the U on top with an airline hooked up to it and I believe that was near silent. However you have to consider that the second drain may pull water faster than the main U tube from the tank? Ie if you have 2 drains larger than the main one , it may get imbalanced . I would suggest whatever you do , you put a gate valve on your main drain. Best thing I ever did I adjust it regularly and I have a similar setup. Without the gate valve it would be noisy. With the gate valve I can keep that water level between the two levels of your drain indefinitely and that's the real key.
Basically I don't see any faults with your plan, except the gate valve is imo absolutely necessary to balance the pump .