There are 2 overflows in the tank. Both have siphons, both have the same function. Both weirs are the same height. And in case I wasn't clear, siphons are 12" below water line in tank/overflow, 1" below water line in sump.
Ok. I think I understand better. If you have two overflows -- is there a third pipe in one of them that serves as the emergency.
Are your valves completely opened? I only get trickle down the emergency when something is is going on with the primary drains (partially blocked, etc.).