The typical reason for 2+ USB cameras to not work together (still they might be working fine separately) is that USB bandwidth is insufficient for them both to run simultaneously. There is a bandwidth limit, which is rather low: The maximum throughput of an isochronous pipe (which is usually used for video) is 24MB/s.
- Implications of using many USB web cameras
- How many USB cameras can be accessed by one PC
- Not able to capture video using 2 webcamera simulteneously
- Two webcams on one usb hub – bandwidth issues