Well actually a Herbie can drain more flow without using the back-up, provided it's done properly. For example if you have a standard Durso and you cover the air hole on the top you'll start a siphon and you'll be able to take more flow, this is especially a Herbie but you fine tune the flow rate with a gate valve to match the return and run a back up drain encase of the perfectly matched flow becomes unmatched. It's not a mystery but rather the effect of head pressure, so ideally the drains are placed at the bottom of the overflow and no extension tubes are used, this will however result in the entire overflow box draining during a power outage.
You can get around the overflow draining issue and you can also use a standard standpipe instead of a straight tube for back up. This will allow you to put a little flow through the back-up resulting a less finicky system and almost as quiet.
This is a system I commonly employed when I did installs, however it's better to have the two drains enter the sump separately rather than joining together first as shown.
