First of all have you seen this thread. It is the origin of the herbie overflow from 10 years ago. it's features are dead silence and flexability. it is designed to run 2 drains in 1 overflow box. not 4 drains in two or two drains in two separate boxes.
NEW plumbing method for an ULTRA QUIET REEF TANK!!!! LONG !!!! - Reef Central Online Community
Next, a 1" PVC
pipe will handle MUCH MUCH more than 600 gph
when at full siphon. I have one on one of my tanks that handles flow from a sicce 5 pump (about 1200 gph
) with the gate valve on the line closed about 1/2 way.
This is the DSA 155 gallon custom tank I got today drilled for a bean animal overflow which is very similar to the herbie. (just the herbie taken another step)
the two outside smaller holes are return lines the 3 in the middle are 1" drain lines. (look for a build thread soon)
The drain lines should not be merged.
you might want to look into a different option for this tank