I had the same issue. Say we have two cameras with serial number 4 and 5. The solution I found to this was to have the camera with higher serial number (5) switched off. Add the first camera. Switch on camera 5. Add the second camera.
For some reason the dcam adapter sees the camera that has a higher serial number and once that is added it is masked from the serially lower camera. Starnge but that is what I understand.
Let me know if this solution works for you as well.
I should note that I am able to access both cameras one at a time; the only
problem happens when I try to add both of them. I also tested another
software HCImage Live. That software is also able to access both cameras
without a problem.