If you have dual overflows the best way to setup a herbie is to link two the identical overflow setups together. Each overflow should have one siphon and one emergency. Essentially this means each overflow needs to be identical. The siphon and emergency can be as simple as a straight pipe from the bulkhead to the top of the overflow. The siphon should go around ¾” of the way up and the emergency should be just above the target water level in the overflow box. With the inside of the overflow done move onto the outside plumbing. Tee the two siphon drains together and then install a single gate valve so the one valve will control the water level in both overflow boxes, pressure and simple physics will insure the levels in both overflows are identical. Next either tee both emergency drains together or leave them separate, doesn’t matter as long as they are both get plumbed to the sump in some way.
To adjust open the valve all the way, set your return pump to the desired flow rate, slowly close the gate valve in intervals and wait for equilibrium. Keep adjusting until the water level in the overflow box is at the desired level which should be just below the emergency drain.
Also the emergency drain size does not technically have to be larger than the siphon as long as if you close the gate valve completely the emergency can keep up it can be any size you want. Personally I would tee both 1” drains into a 1.5” tee and then attached a 1.5” gate valve and use that for the primary but it’s whatever you’re most comfortable with provided it works.
Another option is to use durso or another type of standpipe in place of the emergency drain. It will still function the same, however you’ll notice over time a herbie isn’t that consistent and requires adjustment from time to time. Using the standpipes in place of the emergency will give you a little more tolerance as even if a little water starts to go down the emergency it will remain fairly quiet.
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