Usually, in multicamera systems, you can have an external physical device, sometimes the camera itself to send an external physical signal to initiate a capture. This would ensure that the capture occurs at the same time instant. Even if I use a USB API, I believe absolute synchronization can only be guaranteed from a real-time operating system but not with normal Ubuntu distribution. Such synchronized data is a necessity for multi-view applications like for finding correspondence to estimate disparity and eventually depth. Any slight change in the instance of capture could possibly affect the algorithm output.
I was using two theta S cameras in Ubuntu system, which essentially gets detected as /dev/video0 and /dev/video1. But we were not able to achieve absolute synchronization. We tried to reduce latency and improve synchronization using VideoCaptureAsync class in OpenCV along with ApproximateTimeSynchronizer in ROS. However, this does not achieve absolute synchronization as we have no control over the camera to initiate a capture.