It seems to work for me. I plugged 2 USB camera, here is the kernel output:
Nov 10 16:56:42 linaro-alip kernel: usb 1-1.2.1: New USB device found, idVendor=041e, idProduct=4095
Nov 10 16:56:42 linaro-alip kernel: usb 1-1.2.1: New USB device strings: Mfr=3, Product=1, SerialNumber=2
Nov 10 16:56:42 linaro-alip kernel: usb 1-1.2.1: Product: Live! Cam Sync HD VF0770
Nov 10 16:56:42 linaro-alip kernel: usb 1-1.2.1: Manufacturer: Creative Technology Ltd.
Nov 10 16:56:42 linaro-alip kernel: usb 1-1.2.1: SerialNumber: 2015030235938
Nov 10 16:56:42 linaro-alip kernel: uvcvideo: Found UVC 1.00 device Live! Cam Sync HD VF0770 (041e:4095)
Nov 10 16:56:42 linaro-alip kernel: input: Live! Cam Sync HD VF0770 as /devices/platform/soc/78d9000.ehci/usb1/1-1/1-1.2/1-1.2.1/1-1.2.1:1.0/input/input6
Nov 10 16:57:02 linaro-alip kernel: usb 1-1.2.1: USB disconnect, device number 7
Nov 10 16:57:23 linaro-alip kernel: usb 1-1.2.1: new full-speed USB device number 8 using msm_hsusb_host
Nov 10 16:57:23 linaro-alip kernel: usb 1-1.2.1: New USB device found, idVendor=046d, idProduct=08da
Nov 10 16:57:23 linaro-alip kernel: usb 1-1.2.1: New USB device strings: Mfr=0, Product=0, SerialNumber=0
Nov 10 16:57:23 linaro-alip kernel: gspca_main: v2.14.0 registered
Nov 10 16:57:23 linaro-alip kernel: gspca_main: gspca_zc3xx-2.14.0 probing 046d:08da
Nov 10 16:57:24 linaro-alip kernel: input: gspca_zc3xx as /devices/platform/soc/78d9000.ehci/usb1/1-1/1-1.2/1-1.2.1/input/input7
Nov 10 16:57:24 linaro-alip kernel: usbcore: registered new interface driver gspca_zc3xx
Nov 10 16:58:05 linaro-alip kernel: usb 1-1.2.3: new high-speed USB device number 9 using msm_hsusb_host
Nov 10 16:58:06 linaro-alip kernel: usb 1-1.2.3: New USB device found, idVendor=041e, idProduct=4095
Nov 10 16:58:06 linaro-alip kernel: usb 1-1.2.3: New USB device strings: Mfr=3, Product=1, SerialNumber=2
Nov 10 16:58:06 linaro-alip kernel: usb 1-1.2.3: Product: Live! Cam Sync HD VF0770
Nov 10 16:58:06 linaro-alip kernel: usb 1-1.2.3: Manufacturer: Creative Technology Ltd.
Nov 10 16:58:06 linaro-alip kernel: usb 1-1.2.3: SerialNumber: 2015030235938
Nov 10 16:58:06 linaro-alip kernel: uvcvideo: Found UVC 1.00 device Live! Cam Sync HD VF0770 (041e:4095)
Nov 10 16:58:06 linaro-alip kernel: input: Live! Cam Sync HD VF0770 as /devices/platform/soc/78d9000.ehci/usb1/1-1/1-1.2/1-1.2.3/1-1.2.3:1.0/input/input8
ANd then I could run these 2 commands at the same time.
gst-launch-1.0 v4l2src device=/dev/video1 ! glimagesink
gst-launch-1.0 v4l2src device=/dev/video0 ! glimagesink
What errors do you have?