Conflict between OS and Windows NVMe

Hi,

I have a strange problem, I have Windows 10 on 1 NVMe and OS Catalina on another NVMe.

The issue I have is only one will work at a time in the computer, if both NVMe are plugged in i get a black screen and no USB power.

However if I plug either in separately they will boot up.

Is there a way I can stop them conflicting with each other, they used both work together and i'd tap F8 to select which one to boot from but something has changed somewhere and I'm not sure where.

Many thanks