Weird overflow design? please help

Background and information....

Ok, I got this tank at Inland Aquatics about 10 years ago and it has been in the basement ever since. When I bought it, the guy told me that it had never even had water in it and they were just going to use it for a display tank but never got around to it.

I finally built a stand for it yesterday and just finished filling it with regular water to make sure there were no leaks, and there wasn't

I've always been curious about how these overflows were designed, maybe I'm just missing something. First of all, the tank has no holes drilled in it yet, so I have some options on what to do. The tank is acrylic and measures 6' long x 2' wide x 22" high, just shy of 180g.

There are two overflows, one in each corner, however, only one has slots in it at the top. The other is solid all the way up except for a hole that is drilled in it about halfway up.

The other weird thing is that there is a skinny chamber inside each overflow that goes almost to the top of the overflow. Water can enter this chamber from the tank via a series of small holes that are drilled in the side of each overflow.

I have two return pumps that will supply about 1,000 gph each when installed. I have bulkheads for 2" drains and 1" returns.

So........for the questions

1) Does anyone know what this extra "chamber" is for inside each of the overflow boxes?

2) Should I use one overflow just for draining the water and the other just for return, or should I cut some sort of slots or way for the water to enter the other overflow box and then put one drain and one return in each overflow?

3) What should I do about the hole? plug it, leave it, etc?

Any help and/or suggestions are appreciated, this is something I've wondered about for years.

Here's a link to the photos.

Overflow pics - a set on Flickr

 

Could the one with hole in it be used to house a pump for circulation?

 

Possibly, I have three Koralias that I'm going to use for circulation, so unless I use it for one of the return lines from my main pumps, I have no use for it.

Anyone have any suggestions on running the drains and returns?

 

Anyone else have any suggestions on this? I'm ready to do the plumbing now and I'm kinda at a loss on how to do the drains/returns.

Thanks