Thanks for the comments.
Also, the drains have enough capacity that even if the siphon fails to start they can keep up with all the flow from the pump, it's just noisy.
See this website for a lot more info on this style of overflow: silent-and-fail-safe-aquarium-overflow-system
Nope, the siphon start by itself as long as the end of the outlet in the sump isn't too far submerged, in which case the back pressure may be too much to purge the air out of the pipe.Since you mentioned siphon do you have to start the siphoning to make the tank to drain work ? say like may be a power outage.
Also, the drains have enough capacity that even if the siphon fails to start they can keep up with all the flow from the pump, it's just noisy.
See this website for a lot more info on this style of overflow: silent-and-fail-safe-aquarium-overflow-system