Do they fail when launched in the same gst-launch command ?
gst-launch-1.0 -v v4l2src device=/dev/video0 ! image/jpeg, width=320, height=240, framerate=15/1, format=MJPG ! nvv4l2decoder mjpeg=1 ! nvvidconv ! xvimagesink v4l2src device=/dev/video1 ! image/jpeg, width=320, height=240, framerate=15/1, format=MJPG ! nvv4l2decoder mjpeg=1 ! nvvidconv ! xvimagesink