Problem running the 4th 1080ti's on 2 (1200W PSU's)

Hey fellas, I’m having some issues starting up my 4th 1080ti… so here’s more information

First 3 1080ti’s -> Running on PSU 1 + MB (H110 PRO BTC) + 2 molex connectors (from MB to PSU as required by the MB, otherwise the 3rd 1080ti will not work)
4th 1080ti -> Connected to PSU 2
Both PSU’s connected through a PSU splitter (https://images-na.ssl-images-amazon.com/images/I/61n6cPQClZL.SY355.jpg)
PSU 1 -> Power is directly from the wall outlet socket
PSU 2 -> Plugged into an extension socket (https://media.rs-online.com/t_large/F6963648-01.jpg) along with the monitor power (only 2 sockets being used) -> Could this be the issue?

Some of the troubleshooting I did:
Replaced the PSU splitter (no change)
Checked PSU to GPU connections (everything is connected)
Checked GPU by swapping it with first 1080ti (it worked, so ruled out gpu not working)
Connected the secondary splitter to PSU 1 and primary splitter to PSU 2 (primary was connected to PSU1, secondary was connected to PSU 2, a strange thing happened, all the 4 1080ti led lighted up for a brief second before it stopped working. (no idea what is happening here, it blinks on and off every few seconds)
Both PSU’s are newly bought from amazon (1200W PLAT CORSAIR PSU’s x 2)

I would appreciate all the help and suggestions to solve this problem, thank you!

switch gpu’s, move one that you know works to the other psu and the one that isnt’ working to the side that is…

Does is stay the same or did the gpu that didn’t work on psu2 also not work on psu1? if yes then you have a bad gpu.

If the gpu on ps1 that did work doesn’t work when connected to psu2, then you have a bad riser, power cord, cable, something

Yes, Citric, I already did that. Stated it under the troubleshooting list “Checked GPU by swapping it with first 1080ti (it worked, so ruled out gpu not working)”

but you did not list that you took a known working gpu and moved it to psu2. or that you switched out risers.

also is the riser and gpu that is on psu2 both connected to the same (psu2)

Yes, gpu and risers are connected and powered by PSU2.

One of your image links is broken so not sure what you mean by splitter. If you are using a Add2PSU, then the molex comes from the first PSU and the second PSU’s MB connector plugs into that.

From there, you should run 2x PCIE connectors from the first PSU to 2 cards. Another single PCIE to each riser.

On the second PSU, run 2x PCIE connectors to the other 2 cars. Then another single PCIE to each riser.

That should be a total of 6x PCIE connectors used for each PSU which those models should have.

From there you are completely balanced and you can swap risers, PCIE slots and cards around until everything fires up. One other thing to consider is that you could technically plug everything into a single 1200w PSU to see if they all fire up. Unless they are under an actual load, you could run 4 cards and the MB. For example, I run my cards at 220W and my MB/HDD take very little which would put you under 1000w if you tune correctly for mining.