There also needs to be a big enough gap between the top of the main and secondary standpipes to give the water level some wiggle room. Otherwise it's apparently very hard to adjust. But if you have several inches to play with it becomes somewhat self adjusting. You close the gate valve a bit, the water level rises, which increases the head pressure. Then the increase pressure pushes more water down the drain and it balances out. To a point of course. You can't just crank on the valve.
The basic idea is you use BOTH holes as drains and the return is plumbed over the tank.
The larger hole is used as the main drain with a gate value under the tank. You use the valve to reduce the flow to the point where water stands high enuf in the overflow box to prevent ANY air from entering the standpipe. This is what makes it silent. There's no air in the drain at all.
The smaller hole is used as an emergency drain in case the main drain gets clogged. This is very important as the main drain's flow is intentionally reduced with the gate value. The secondary drain should be plumbed completely separately all the way to the sump and does NOT have a valve.
Here's a link to a post with a picture. The water level shown in the picture isn't exactly right. My understanding is the flow in the main drain should be adjusted so that the water level doesn't actually reach the emergency standpipe. Or if you do use the 2nd one, it should only be a trickle.
