Hello
Not exactly
I Build a farm for a client with 64 machines in the litecoin era, using 2 bridged PSUs (sentey 850W SS series (superflower design)) and worked perfectly, not 1 of the power suppies failed nor the GPUs after two years, we sold all by now (I still have like 15 of those power supply, bought in 2013, and stil works perfectly.

there are several secrets for this:

1) the power supply must be identical (brand, model and power rating and 80+ rating).
2) you need an add2psu gadget (i made my own, which is basically a reed relay, when it senses 5V it turns the second PSU on)
3) you need dual conversion PSUs (usually found only on PSUs above 900W or so) what i mean is PSUs that converts the entire energy into 12V single rail, and then uses VRM to generate 5V and 3.3V, the cheaper PSUs converts using different taps in the transformer for each voltage, that ones dones not work, because the second psu (which does not power the motherboard, only the GPUs) had the +5V not loaded, and since those cheap PSUs regulates over the 5V line, and not on the 12V line like the dual conversion models,  the voltage keeps going up and down all the time, eventually fucking something up or the PSU itself.
So the most importan factor DUAL CONVERSION PSUs is a MUST. they are easy to spot , if you see a vertical VRM board with big coils, then it's dual conversion and it is good for dual PSU usage.
4) and finally, the first PSU MUST power the motherboard and ALL OF THE RISERS, this is extremely important, because the risers voltage shares the line with the PCI-Express bus and if there is a tiny litte difference in voltage all hell break loose.

the second PSU powers the 6 and 8 Pin conectors on the GPU (i also power the first GPU with the first power supply) to balance the consumption between the GPUs, you have to measure current with a clamp meter that can read DC AMPERES (not many can do that, i have the APPA A18+ clamp meter that can read DC AMPERES perfectly) so measuring you know how much each psu is being loaded.

if you need any specific data ask me, i will be glad to help.
cheers
indkt.