Is it possible to trigger two Cameras connected by USB to a single RPi at the same time with 1 ms latency?
If you have the right Canon cameras you can use the Canon Hack Development Kit CHDK. I use that and StereoData Maker to trigger two cameras. You turn the 5v USB pin on to trigger the camera so you need to convert the 3v gpio to 5v.
Try something like;
streamer -c /dev/video0 -b 16 -o a.jpg & streamer -c /dev/video1 -b 16 -o b.jpg &
& makes the second command run without waiting for the first one to finish.