Why two power supplies? How are 3 modules connected to two power supplies? Are they producing redundancy? If so how is that achieved? Is each power supply connected to each board (would imply two power connections pwe board)? Wouldn't one large power supply be sufficient, cheaper, and take up less space?
For our mining rigs we have chosen to specify power supplies that are rated 20% higher than the highest wattage we expect even including significant overclocking. The GN chip uses 250W at nominal, and we have designed all the systems to handle upto 350W. When the mini motherboard is supplying 350W to the chip (this level of overclock should result in up to 540GH/s) the mini motherboard may consume up to 411W of 12v, due to losses in the VRM. Add 25W for fans, and you get to 436W per mini-motherboard. Multiply by 3 chips in the Sierra, and you get about 1300W. We've added a safety margin of 20% extra on top of this - so we needed power supplies that were rated to a minimum of 1560W @ 12v to meet these specifications, and maintain the headroom and margin we desired to ensure reliable, efficient long term operation.
There are very few manufacturers who make single 1600W supplies, and we didn't find a single unit from a high quality vendor. In addition this size of supply is more expensive than 2 smaller supplies. The Sea Sonic supplies are really well built, and even significantly overbuilt. In all tests they are capable of supplying power well above their ratings, and the output regulation is very accurate and smooth. They handle hot operating environments very well. Please go read the reviews of the supplies in the Sierra - the Sea Sonic X-850FM3.
http://hardocp.com/article/2013/04/04/seasonic_xseries_x850_power_supply_review/9#.UmDCKVCcc5ghttp://www.kitguru.net/components/power-supplies/zardon/seasonic-x-series-850w-km3-power-supply-review/7/Each GN chip is on it's own mini-motherboard, and each mini-motherboard has 2 separate 6 pin PCI-E power connectors. Each of the supplies feeds 3 of these connectors.