Have you already checked if it could be faulty risers? That'd be the first thing I'd look at.
If the riser isn't the issue it could be that you're not getting both of your PSU's working together. When running multiple PSU's in the same system you either have to jump start the second PSU with a paper clip or get one of these cables -> https://www.amazon.com/Main-Second-Power-Supply-Cable/dp/B00JIV9BRK

If it's a software issue I'm of limited assistance, I'm more familiar with Windows than Ubuntu.