It sounds to me like the first PSU has the capacitor problems I suggested.
PSU with bad, overloaded, or cheap capacitors will allow noise (voltage fluctuations) to pass to the motherboard. One of the many possible results of that is random shutdowns. It can also cause overheating as the caps and MOSFETs on the motherboard are working harder in trying to clean up the dirty power.
There is another possible problem with the Enermax Galaxy PSU.
Enermax and some PSU manufacturers have taken to adding Ferrite Beads to the legs of the MOSFETs and Diode packs inside the unit.
(Photo linked to below shows parts from an Enermax Galaxy 1000W.)
http://www.hardwaresecrets.com/fullimage.php?image=4981Note the little black 'tubes' on the legs of the IC chips.
The problem is the glue used to attach these becomes conductive after heat hardens it. If the assembler is sloppy there can be 'stringers' of glue that connect the of the IC's together and that will eventually short out areas of the PSU internally and cause permanent damage.
Enermax HAS had problems with sloppy assemblies.
Next photo link is parts from an Enermax Liberty that did in fact fail due to this glue.
http://www.badcaps.net/forum/attachment.php?attachmentid=7870----
#1: It would NOT be recommended to use 2 PSU's to power a motherboard and any card attached to it. You risk creating power 'loops' which can cause oscillations in the power on the board. The values of the coils and caps in the output filters of the PSU's may result in creating an oscillator. (If one PSU is at 12.5v and the other at 11.5v you would have a 1v loop bouncing power between the internals of the PSU's through the board. A 0.1v difference may actually cause problems.) Besides the potential for creating oscillations it can muck up the action of the voltage regulators in the PSUs. They will each 'see' the output of the other PSU mixed with their own and each will be trying to adjust their voltage to fix the output of both PSU's. This causes a condition called 'hunting' where regulators are continually adjusting up and down because they can newer find the 'sweet spot' that makes them happy.
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If you are going to safely run PSU's in parallel to the same load you need to add a diode in EVERY wire to the load. That 'should' stop oscillations and hunting.
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You could safely use a second PSU to supply drives or other parts that do not also pull power from the motherboard but you would want to make sure both PSU's and the case are solidly grounded together. (Use a separate wire and ground them all physically. Don't count on internal grounding to do it.)
#2: You can't use dual ATX power supplies without modifications.
- You'd need to fake a +5vsb load on one.
- Fake a 'power good' signal to one.
- Create a way for both to respond to 'on' commands without creating excess current though that circuit on your motherboard. (It's not just a switch w/ATX.)
- And ensure they both meet minimum load requirements. (Or they will turn back off right after they start.)
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All of that would require a new question here on EE.
It's off this topic and to do it is rather involved.
#3: AT power supplies don't need those mods but you'd have to make some mods to get it to turn on/off with the other PSU (or deal with two power switches).
An old style relay switched by +12v from the ATX PSU output and used to switch the AC power to the AT PSU would take care of it. (You would still need diodes if running them in parallel to any load.)
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