Anything that plugs into the wall can kill. So there are always risks.
But in this scenario, assuming the use of dual PSUs is done correctly with undamaged, properly functional PSUs, those risks are minimal and not likely to one's physical safety.
The ATX Form Factor Standard allows for a tolerance deviation of ±5%. One PSU could be outputting +11.5V while the other +12.5 and technically still be in total compliance with the requirements. Yet that 1V "
difference in potential" to a graphics card, for example, would mean +11.5V being supplied through the PCIe slot from one supply and +12.5V through the aux power connector from the other supply. which "may" (I did not say "will") result in stability issues for the card.
But the bigger, and more common issue deals with a "
common ground" (or rather, the lack thereof) resulting in EMI/RFI. It is important to remember that no two wall outlets provide the exact same resistance path to Earth ground. The resistance through the outlets themselves will be different, but also the distance (and thus resistance) from the outlets back to the facility's Earth ground point in the service panel will different.
Plugging two high powered PSUs into the same wall outlet ensures a common ground, but could overload the circuit and trip the breaker - especially in older homes/facilities that may only have 15A circuits. For this reason, some users end up plugging their supplies into two different outlets. And that may result in a difference in potentials between the grounds.
The most common symptom (if any) is "noise" - as in electrical noise, perhaps a buzz in the audio, or lines on the monitor display or in a nearby TV. Another symptom is actually feeling a small "tingle" of an electric shock if you touch the PC case with one hand, and that second PSU case with the other.
I would NOT assume that would be safe if you had a heart pace maker!
The solution (preventative action) is actually quite simple and totally effective. Mount everything in a single, grounded equipment rack. But of course, that is not practical for most home users. So, simply attach some basic
22 AWG "Hookup" wire to a case screw on each case, ensuring the wire contacts bare (unpainted) metal, will establish that common ground.
BTW, if you have a home theater system where your audio components and video components (or big screen TV) are plugged into different outlets, creating a common ground with hookup wire is a common practice to eliminate/prevent interference issues. Same with Ham radio equipment.
Now I am also assuming the wall outlets are properly wired and grounded too. So every home and every computer user should have access to a
AC Outlet Tester to ensure the wall outlet is properly wired
and grounded to Earth ground. I recommend one with a GFCI (ground fault circuit interrupt) indicator as it can be used to test bathroom and kitchen outlets (outlets near water) too. These testers can be found for your type and voltage outlet, foreign or domestic, (like
this one for the UK, or
this one for German outlets) at most home improvement stores, or even the electrical department at Wal-Mart. Use it to test all the outlets in the home and if a fault is shown, have it fixed by a
qualified electrician.
@80251 - in addition to all of the above, I would first look at your heat and cooling situation especially since you say this only happens when playing your most demanding games. You definitely need to eliminate heat as the problem before anything else. Otherwise, you may have a nice, dual-PSU computer that still locks up due to heat.
If the interior and vents are all clean of heat trapping dust, and all fans are spinning properly, open the side panel and blast a desk fan in there while playing your game to see what happens.
