Osama...
I use a Sequence 4600 (1/6 hp) as the sole return from my sump, which is fed by 2x 1.5" drains. I bought this pump to replace a Dolphin high RPM, high head pump that was too loud, too hot and used too much electricity (I think it was the model 4200, and used something like 6 amps). The Dolphin pump was great, but was better suited to applications requiring much higher head (e.g., pumping water to the first floor from a sump located in the basement). This pump also tended to cavitate because by estimated head of 12' was no where near the pump's head capacity of 30'. If I had a 2" bulkhead in my sump, I would have also looked at the Dolphin Ampmaster model no. 5600 (or something like that). The bottom line is that the best sump return pumps are low rpm, low head pumps. These pumps typically use large (1.5"-2") intakes and outtakes. For that reason, I would suggest using 2" bulkhead fittings or include extra 1.5" bulkhead fittings on your sump so that you can take advantage of the bigger pumps if you ever decide to upgrade.
The other 2x 1.5" drains (I have 4 total) are each connected directly to a pair of Iwaki 40RXLT pumps plumbed in closed loop fashion. Thus, the water exiting my overflow chamber through these 2 drain bulkheads does not drain into the sump, but straight to the Iwaki pumps. The Iwaki pumps returns the water through a pair of 1" SeaSwirls placed at the front corners of the tank so as to sweep back across the front of the reef structure.
Placing the drains for the closed loops inside the overflow chamber has advantages and disadvantages.
On the plus side, I can get to and replace, if necessary, the bulkhead fittings without draining the tank. In other words, if one of these bulkheads starts to leak, the tank will not drain since the bulkheads are inside the overflow chamber and isoslated from the main tank. This is in contrast to those that place their closed loop intakes through the back wall of the tank and separate from the overflow chamber. In those set-ups, one would have to drain the tank below the level of the intake before working on them.
It is also easier to get to the intakes (for cleaning) when inside the overflow chamber as opposed to along the back wall where access to the intakes would most likely be obscurred by live rock.
On the negative side, all of the water that is being recirculated within the tank ends up passing over the overflow partition. Thus, you have to make sure the overflows are big enough. Mine is about 20"x6", for a total partition length of about 32". The depth of the water passing over the walls is about 1/2". This amout of water flow can be pretty loud.
The other negative is that air bubbles created by the water flowing into the overflow chamber tend to get sucked into the closed look intakes. However, this is a problem that can probably fixed by the use of well designed Durso standpipes to minimize the distance the water falls into the overflow chambers. You could also look into adding some baffles inside the overflow chambers.
One final possible negative is that if your sump return pump fails, your overflow chambers MAY drain down to far to prevent your closed loop pumps from running dry. In theory, if your sump return pump quits, the water level in the overflow chambers will drop below the top of the durso standpipes that are connected to the bulkheads that feed the sump, and water will stop flowing to the sump. However, there should still be enough water in the lower half of the overflow chamber to feed the closed loop bulkheads (which do NOT have Durso stnadpipes). Ths water being circulated by the closed loop pumps should still enter the overflow chamber, fall to the top of the water level in the chamber (which should be below the sump standpipes), and feed the closed loop intakes unabated.
In any event, your plan to use a pair of cormer returns, each measuring 12"x6" and including 2x 1.5" drain bulkheads sounds fine to me. This will have slightly more flow capacity than my set-up. As for closed loop pumps, I would probably use a single Sequence or Dolphin pump as opposed to the pair of Iwakis I am using, but it makes no practical difference which way you go.
