There was an earlier thread with your same problem. The answer to that one was at least one of the power supplies was bad from the manufacturer. I do believe that OP did what you suggested at the end of your first post.
IF you suspect manufacturer defect, will you be trying to send them back for refund or replacement? If not, you can open one up to see if you can find anything obvious, like a cap that is bulging or leaking, or bad solder joints. Quality these days from online suppliers has been subject to sketchiness.
The rough thing about these is that they are switching supplies, not some beefy linear supply with a massive transformer. As such they run the possibility of, as you’ve already explained, being off frequency or out of sync, which causes a beat frequency when you have a common connection between the two.
If the frequency (for you) is either 50 Hz or 100 Hz, then that points to power line issues and a ground lift may help.
Or, if you can add another larger electrolytic cap and maybe a small ceramic cap near the output of the PS. These will help quell ripple and RF.
