One thing you might want to consider for the box with your returns. Since there will be no flow inside the box you will probably have some stagnant water in there due to lack of circulation. You could either put a powerhead in there to help circulate or you could fill it with sand, creating a deep sand bed. This is how Bean himself approaches his system on tanks with dual corner overflows. Just something to consider.
I'm having my new tank built with dual overflows as well, but I'm having three holes in each box, (2) 1 1/2" (1) 1". The 1" will be my returns since I need this tank to be flush against the wall, and each box will have a full siphon 1 1/2" drain that will join together, Herbie style, under the tank into a single 2" line with a 2" gate. The left box will be my emergency and the right box will have an 1 1/2" standpipe that will serve as a dry chamber for all lighting cables and such. Lots of guys on reefcentral have run two full siphon drains connected to a single line and gate with great results. Not saying you should, but it seems to work well since one gate controls the water level in both boxes.
I'm having my new tank built with dual overflows as well, but I'm having three holes in each box, (2) 1 1/2" (1) 1". The 1" will be my returns since I need this tank to be flush against the wall, and each box will have a full siphon 1 1/2" drain that will join together, Herbie style, under the tank into a single 2" line with a 2" gate. The left box will be my emergency and the right box will have an 1 1/2" standpipe that will serve as a dry chamber for all lighting cables and such. Lots of guys on reefcentral have run two full siphon drains connected to a single line and gate with great results. Not saying you should, but it seems to work well since one gate controls the water level in both boxes.
