Only one card at the time is detected

Hi everyone,
I know the subject of this topic seems to be quite widely covered, but I can't find anyone having the exact same problem as me and I was wondering if any of you had an idea of what I am doing wrong. Thanks in advance for your help !

I started mining a month ago on a single RX570 and made enough bitcoins to want more ! I then looked at GPU prices and decided to add a 1060 3Gb as I read somewhere they are slightly better than RX570 (this could be wrong though). My motherboard is an Asrock B250M-HDV and I only have one x16 and 2 x1 PCIE lanes so I bought a riser. Since my psu is only 430W I added a 300W PSU using a connector so both start at the same time.

If I plug the RX570(principal 430W PSU) in the x16 and the 1060(second PSU) in the x1 using the riser, the 1060 is not recognized. However, if I disconnect the RX570, I am able to use the 1060(powered by the second PSU and plugged in the x1 lane). I think this means that my second psu, the riser and the 1060 card are all working properly. My hypothesis is that the mobo is the problem here (I can be wrong !)

I tried different combinations of GEN1/2/3/AUTO in the UEFI with no changes.

Have you any ideas ? I am forgetting something ?

Thank you again for helping noobs like me getting into mining.

AL