[sphelps]

If it was me I'd run the return separate (not through the overflow) allowing the 4 holes to be used for the drain system and realistically better options for the return as well.

The four drains setup as follows:

The smaller hole from each overflow, tee together then to a single gate valve, from there to desired sump section, this would be the primary drain. This primary drain will be the full siphon, so extend a pipe in the overflow towards the surface as needed to prevent the overflow from draining completely during shut down. Just don't make it too close to the surface otherwise you'll get a vortex and introduce air into the drain. The single gate valve will actually allow better constant control over two separate valves as the flow rate to each overflow may not be constant but the overall flow rate will be. Hence with two gate valves, one for each overflow, you'll constantly be fighting with them to reach equilibrium only to discover it's not possible.

The two large holes again can be tee together to the desired sump section, or they can be maintained separately. Use standard durso standpipes for each hole, setup just slightly above the desired overflow water level or at the level they will receive a slight trickle at optimal overflow level.

Adjust gate valve so the dursos receive a little water for best results, it will allow more freedom and less constant tinkering. The result is a cross between a herbie and a bean animal, after all when it comes down to it the bean is just a herbie with an extra standpipe, really the same concept. The advantage here is both overflow boxes will get used so no dead zones, two dursos act act as back up which should handle full flow during primary clog. Using actual durso standpipes rather than straight pipes for back up allows the ability to send some flow down them without major noise issues and if the flow path changes some how there is extra freedom to prevent the need for adjustment.