So it sounds like you have a Herbie style drain. Look at the picture closely on the marinedepot blog post. If shows you height and flow you want. In the readings it really tells you how the valve works in dialing in your flow to achieve the correct water height.
But essentially the one with the valve is your main drain. you want that thing with most all of the water flowing through it, you want just a small trickle if any at all through the second "emergency drain". This is there the valve comes in. Keep in mind you have to balance your return pump so that you're not sending so much flow into the DT that just one pipe can't handle it all.
