Remember, if you do use 2 PSU's... you WILL need something on both PSU's to give the PSU a 5v load. Without something on the 5v rail, the 12v rail will not have a reff-voltage to stabilize correctly.
EG, plug an old CD-Drive or Floppy-Drive or any other device into one of the standard MOLEX (four prong "oooo" cables.) For the AMT unit PSU, this would be the raspberrypi as the 5v load. For the other PSU, you will need another device. (Find a nice 5v LED-light display, and you have a cool load to light-up the cards.
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P.S. An external 12v fan, will not work, as it does not use the 5v on the molex connections, though it may have a wire on the plug, it is not actually connected to anything. (It will say 12v on the fans sticker.)
You know if you have power issues, because some boards will run at abnormal speeds. If you turn the unit off, and remove a board, and the remaining boards run normal... or better... then you are running into voltage issues. The PSU is not providing enough AMPs, while it tries to maintain the correct VOLTs. (Thus, the stated above solution, or you need a PSU with more AMPs on the 12v rails, to match the cards draw.)
When buying a PSU, you want to mind the RAILs and the AMPs per rail.
+12v/+12v (2 rails) *may not be split correctly for 5 cards, 2 @ 30a each (great), 3 @ 20a each (starved)
60a/60a
+12v (1 rail) *better for 5 cards, 5 @ 24a each (ok)
120a
NOTE: Without something on the +5v load, you may only have 50-80% of that amperage on the 12v rails. Voltage will actually drop to 10.5v-11.5v once the unit starts running, on the weak rail, in a multi-rail PSU. That, or it will remain 12v, but be lower available amps then expected. Only 30a-45a per rail, in the dual-rail setup mentioned above.
That is just an example. I will know more detail about each cards individual "ideal power consumption", as my unit arrives for testing.
Good info reenforcing what was seeing as I was checking out PSU's.
For the latest PSU's look for 'certified for Haswell' and/or the equiv AMD power spec. Basically means they are just fine with no load on the 5v rails to work with the latest near-zero power cpu sleep modes.
Also again, double check the power specs vs your power line voltage. On several 'consumer' PSU's over 1,200w I saw that many (not all) only give full power when fed from 220v. Their 110v ratings are much lower. For all server PSU's over 1,200w I've looked at that is 100% the case.