Overflows - are 2 needed on a 125 gallon tank????

@ are not necessary, one will be adequate. You can get the desired turn over rate with one. Depending on what is being kept in the tank (e.g. SPS) you can just use powerheads in the tank to get the desired flow in the display.

 

2 is best, but you can probably get away with one if big enough.

You could always drill it & have the overflows on the back side

 

I have 1 on my 150. I highly recommend looking into the beananimal system though.
http://www.beananimal.com/projects/silent-and-fail-safe-aquarium-overflow-system.aspx

 

One is fine for filtration. The longer water is able to flow through the sump and fuge, the better it is filtered than if it was rushed through at max velocity. Though with in reason, in that happy medium of not too fast and yet not too slow.

If you need more flow with in the tank, like said earlier, upgrade your powerheads.

 

I'm running an estimated 700 gph myself

 

Bella is right on with a single overflow. If you are going to drill a hole in the back of your tank and diy, dont forget about that durso standpipe talked about in one of the other threads. That will be a long drop to an elbow or whatever for the water. I have never had a problem with a ball valve at the end, so i can tweak the water level some more to quiet it down, but that's risky if something got caught in it as well.

 

this question greatly depends upon the load/critters you will have in the tank. as for filtration, you can never have to much available (more is better), a clean tank is usually a healthy tank, therefore, I prefer 3 or 4 overflows in a 120 (or any 5 foot or larger tank- one on each end and one or two in the center). this allows for many options, and also will greatly reduce "dead spots" with-in  the entire system. it also allows you the freedom from internal power heads because you can use the return pump(s) from your sump(s), as well - all the while, using a wave controller on the return pumps.

 

I use a beananimal overflow and love it. If it is an option I would look into it. I have a single aga overflow in a 90g and it works fine with around 700gph. William

 

well on my 165 tall, I run four returns from my sump and three overflows, the corner overflows are on on a shared 1400 gph return pump tee'd off at equal distance on single flex return lines, and the center is on a 900 gph pump with a double flex return line. this gives me nice flow and is directional- no need for internal power heads, all three returns/overflows have ball valves (7 total) and are on a wave timer set at 20 sec intervals. this really removes all the clutter from inside the tank, a look I very much prefer.
edit, btw, when I  do my 240 gallon this winter I am going to place a sheet of acrylic inside the entire length/height of the tank that is 2.5 inches away from the back wall, this will serve as my entire tanks overflow with four 1.5 inch drains, it will have 8 returns dived equally from 4 return pumps- all this equipment contained in a 75 gallon sump.

 

that beananimal set up is pretty slick, I wonder if they could do the same for my design?