I only have single herbie, but spent a little time googling when I saw your posts a few weeks back.
My suggestion (and I may be wrong): Set one side siphon to fully closed. Open the other side 100%, and turn on your return pump. Adjust the open siphon down so that it gets to a point where both emergencies run quite a bit of water through them. Now open the closed siphon slowly, and bring it to a point the overflows only take a little more than a trickle.
I don't think this will truly "balance" the flow between both sides but I'm not certain you need a true balance. As long as you have movement in both overflows you shouldn't have issues.
Also something to consider is to close both siphons before you start and let the return run for a while under heavy supervision. Make sure your emergency drains are capable of handling 100% of the flow.