I am attempting to capture dynamic streams from multiple usb webcams using only mathematica. Using ImageCapture[“Device” → $ImagingDevices[], I’ve tried to open multiple instances of Mathematica using ImageCapture on different devices for the different instances. No Luck 
Is this too complex a task for Mathematica alone?
I’ve had some trouble with this, too.
I haven’t tried in a while, but I would suggest separating the webcam streams in multiple kernels, or multiple processes, or something.