Antec Signature SP1300 80+ Platinum - Dual PSU setup - not booting 2nd PSU

[jw2k_fr]

I have an open chassis rig with Asus Z590-P
2 x Antec Signature 1300 PSUs
Three GPU plus MB on PSU1
1 GPU on PSU2
Antec OC Link cable connects both PSUs

I have had the machine running fine for 12 months on 1 PSU
added a fourth GPU on a PCIe riser connecter which has a USB connector back to a PCIe-x1 slot

When the power button is pressed, all the GPUs on PSU1 come online
all riser cards with GPUs attached have LED status lights powered up, all 3 GPUs power on
Motherboard on PSU1 also powers up
GPU on PSU2 does not power up, riser card also does not show power lights

I have tried the hybrid mode button both ways on the back of each PSU
<EDIT> This is a rebranded Seasonic PSU and from their site have learnt this button controls fan behavior. </EDIT>

No online reviews I've seen cover dual PSU setups
Antec Support is AWOL - think twice before buying their products!!!

Anyone have a similar setup and know what I'm missing?
<EDIT> I know that I could use a PSU jumper to jerry rig things, but I'm looking to try and get things working properly </EDIT>

Thanks in advance for any help