You make drains same height as the overflow box height will determine water height, I have my box as high as I could put it "touching the middle brace", I didn't get/use a lid as there's no need Sent from my VS995 using Tapatalk
I'm guessing the first drain is running on a siphon if the second is an emergency correct? 1x1" siphon will drain as much as 2x1" standing drains... I'd still go with 2x3/4" for the returns. No sense in these being the limiting factor. Manage flow by pump size