Pages such as this discuss wiring ATX PSUs in parallel, in what seems to be a sensible manner, with links to various projects showing details.
Companies with a decent reputation for ATX PSUs, sell cables specifically for that purpose, and many others sell them on Amazon.
I'm interested in running two PSUs in parallel but would like to know how safe it is, if done right.
I'm running a large (20+) HDD array on an underpowered but extremely high quality 750W PSU. I need to upgrade the PSU to ideally get around 5v @ 35-40A and 12v @ 115-130A. Dual PSU with as second 750W - 850W PSU seems to have a lot of advantages over replacing with a 1600W or higher PSU, and perhaps no disadvantages. Both would be same brand, possibly same model, single rail, and very high quality build+output.
(OP updated: image added as recommended by comments/replies)
Electrically it looks simple enough. Give them both the same shared "soft start/stop" feed, and crosswire 0v to give them a common ground plane. I'd have a stability advantage over the mining users in that I don't want extra power for PCIe graphics cards, my need is extra 5v/12v power with a common ground plane just for external HDDs. So I'd have one PSU handling the baseboard and 1/3 of the HDDs, and the other PSU handling the other 2/3 HDDs.
Parallel PSU also beats a larger monolithic PSU in other ways that benefit me: I need disproportionate 5v, more than even a 1600W PSU would provide (5v @ up to 36A needed for HDDs but not used by PCIe/graphics cards which large PSUs target, so max on even a 1600W PSU is about 24-30A while dual gets me 48A). It doubles my HDD connector sockets so I can reduce current/heating in any given string (5-6 HDD connections per PSU, so only half the number of HDDs on each). It also spreads heat dispersal between them. I don't have resources for a formal dual-PSU case of the kind I'd need, so it's either dual high quality standalone PSU or large PSU. So I'd like to do it if I can.
In theory I'm guessing there shouldn't be much scope for problems due to current loops. Also, unlike wiring in series (see this question), their 0v planes would be connected so even if both PSUs were earthed, I guess the earth would be in common for both anyway, which might help avoid earth loop issues.
In theory, provided it works electrically, I use very high quality PSUs from the same brand/model range (which I would), and I double check the crosswiring and junctions on any crosswiring cable before using it, dual PSU is a far more efficient solution, gives several electrical benefits, and costs 1/4 the price of a monolithic 1600W PSU while guaranteeing that both 5v and 12v peak current capability are met.
But electrically, the PSUs weren't explicitly intended for dual use so I don't know. How risky is it? How safe if done right?
Is this pretty much how any dual PSU works?
Am I missing any other important safety/setup issues that I have to know/do, to dual PSU safely and not imperil my equipment?