I like the idea of Dual PSU, especially for expansion and failover
A Seasonic PSU would be the last thing to fail in this or any miner. I can almost guarantee you that HF aren't using two for failover purposes.
Correct, we're not using two for failover purposes, we're using two because we chose to provide a 20% safety margin on top of what the Sierra might need.
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.
Now that's the real definition of margins on margins.