+ 1False. Two drains with a tuned gate valve will forever be trying to balance itself out. Balancing = noise. One drain for full siphon (gate valve tuned) and one open channel with NO gate valve. The other component a few folks seem to forget is the access to the water. You want the full siphon line to be the lowest level. Open channel an 1" or two taller and the emergency line just above that.
Primary Full siphon with valve, second open channel without a gate valve (position downwards and slightly above the full siphon inside overflow) , third emergency line upwards inside overflow. Adjust the valve on siphon line so that there is just a trickle going through the secondary line, and its able to keep the noise levels in check.
So your gph mostly be from the full siphon only and may be a plus 10% from secondary line.