Thanks for reporting back with these results!
The one screen that goes black - is it consistently one adapter? or both have done it at some point in the past?
I think your tests have pretty much ruled out not having enough USB bus power.
Since the problem only seems to happen when 2 adapters are connected, let’s try to rule out a problem with total desktop/framebuffer size. Can you run a quick test where you 1) plug in one adapter, then set its mode down quite low (something like 800x600) 2) plug in the second adapter, also set its mode low.
Did these steps avoid the problem? (unlikely, but will help rule out that problem).
At this point, what I’d like to do is get you a 3rd test adapter, to see if we can identify one of the units as a troublemaker. Can you email your Amazon order # to email@example.com, and let us know if the address from that order is still good.
We’ll get this figured out and solved. Thanks so much for your patience!