Sharing video from second camera disables first camera

Hello -

I am a teacher using Windows 10 for Education on a Lenovo ThinkCentre desktop with a built-in webcam. I have also connected a IPEVO V4K document camera. Zoom recognizes both cameras - I can use either one as my primary video feed and both work properly. However, I cannot get the "Share content from second camera" screensharing feature to work correctly. 

If I am using the built-in webcam and select "Share content from second camera", the second feed (document cam) is black. If I *turn off* my main video, then select "Share content from second camera", it shares fine. But as long as the screen sharing is active, I am unable to turn my main video back on (so my students can't also see my face and gestures). 

The same problem happens if I reverse the sources -- that is, if I make the document camera my primary video, then I cannot screen share from the webcam. But if I turn off the primary feed, I can screenshare from the webcam -- but my main video remains disabled until I end the screenshare.

To be more precise, while screensharing from one camera, I cannot turn *the other camera* back on. If I switch the feeds so that both the primary video and the "second camera" are actually the same source, it is able to show that feed both in the screenshare and in the main video box. 

Has anybody encountered this or do you have any suggestions?