When watching videos played during the meeting, I watch my computer or TV so that I see and hear the video as my students do. I am teaching things that require split second timing. There is a 1-3 second delay in the video for participants as it goes through Zoom and to their computers/devices. By watching the video on a second device, I see it exactly at the same timing as my students.
I often use a cut-off headphone jack on my tablet during meetings to avoid microphone/speaker feedback between the two devices & the TV. This is easier than remembering the specific settings on the various devices to avoid it. Pulling out the headphone jack however did not correct the issue described above. And does not cause a problem when Sharing videos via mobile Firefox.