There is ONLY one potential issue, with relation to "plug-neutral"...
This "situation" only exists if you have one PSU plugged-in to one half of the circuit-hot-A, and the other PSU plugged-in to the other half of the circuit-hot-B. The circuit I am talking about is the two circuits which US power is split into. Usually, each room has multiple plugs, but all are either on circuit-half-A or B, not both in one room. However, if you had an "extra" leg of power added to a room, and that leg is the opposite half of the hot-circuit... and you plug one into one, and one into the other, and attempt to join them... there is a 50% chance that the unrectified connection will share a 220-240v potential between the PSU's, through the hot-legs. Since the "common" is grounded to "neutral", which is shared for both, since they are both mounted in the same case, and grounds for 12v are shared "dc common", throughout the motherboard.
This is NOT an issue if you are using one power-strip for both PSU's. But I imagine someone my hijack an additional outlet, for use in bitcoin mining, after having a second-leg circuit installed into a new room, along side of other equipment.
You can join any grounds, since they are all already joined by the case, the MOBO, and every other component. It is "better" to have the cases themselves grounded, as that stops the "noise" from traveling through the ground-plane in the MOBO, where the cases are not joined/grounded. If they were isolated, the cards would not function. Only optoisolators could be used to separate the two sources, which is NOT how any computer components function. (Only old parallel ports did that, where voltage-potential "leaking back in" would destroy the sensitive LPT chips.)
Voltage will never "FLOW" back from one PSU, into another, due to all the circuit designs which protect against that. You will simply get the average voltage potential out, and the combined amps out, for that average voltage.
12V + 10V = 11V out (On a starved PSU that is outputting 10V for whatever reason.) The combined amps would be 10A and 8A = 18A @ 11V. Thus, more stable than one PSU, which would be that one starved at 10V, which would drive-up the amps, and further kill the PSU or attached component. 2V will not be "flowing in" to the other PSU, unless there was no load attached, and unless the circuits had reached the reverse-voltage break-down limit, which would not happen with 10V on the other side and 2V "trying" to potential-in.
This is NOT against ATX V1, or V2 specs... this IS ATX specs. It is called tandem and redundant operation supplies of external sources.