I have been working multiple Astra Orbbec Pros for a project and I ran into the same problems you describe. I created a similar setup using just Astra’s SDK. While multiple sensors work there, the Color streams for any cameras hooked up beyond the first one do not work and show a black screen. Depth frames and skeleton tracking work as expected.
Back in Nuitrack, I tried disabling the color stream based on information from the FAQ (“How can I disable the RGB stream?”) and this fixed the problem for the 2nd sensor. Of course this is a problem if you still need the Color camera. I don’t know if this is a recent issue with the Astra sensors because there is a sample in the Astra SDK that displays views from multiple sensors, color and depth, but I have not been able to replicate the screenshot in their user guide in my testing.
Hope this helps.