open 2 USB cameras simultaneously on VLC player Ubuntu 12.04 - opencv

Question

How I open a Camera:
In terminal (text after the $ prompt = commands that I type):_________________
$ ls /dev/video*
/dev/video0 /dev/video1
$ vlc v4l2:///dev/video0
VLC media player 2.0.8 Twoflower (revision 2.0.8a-0-g68cf50b)
[0x9f2d908] main libvlc: Running vlc with the default interface. Use 'cvlc' to use vlc without interface.
VLC starts playing camera output.
In another new terminal (BOLD = commands that I type):
$ vlc v4l2:///dev/video1
VLC media player 2.0.8 Twoflower (revision 2.0.8a-0-g68cf50b)
[0x9b24908] main libvlc: Running vlc with the default interface. Use 'cvlc' to use vlc without interface.
[0xb0500960] v4l2 demux error: VIDIOC_STREAMON failed
libv4l2: error setting pixformat: Device or resource busy
libv4l2: error setting pixformat: Device or resource busy
libv4l2: error setting pixformat: Device or resource busy
libv4l2: error setting pixformat: Device or resource busy
libv4l2: error setting pixformat: Device or resource busy
libv4l2: error setting pixformat: Device or resource busy
libv4l2: error setting pixformat: Device or resource busy
libv4l2: error setting pixformat: Device or resource busy
libv4l2: error setting pixformat: Device or resource busy
libv4l2: error setting pixformat: Device or resource busy
[0xb0501188] v4l2 access error: cannot set input 0: Device or resource busy
[0xb0501188] v4l2 access error: cannot set input 0: Device or resource busy
[0xb5300618] main input error: open of `v4l2:///dev/video1' failed
No video is played, only error message.
Primary Objective: I want to open two cameras simultaneously in opencv (c++) ... got similar errors with opencv .. so I'm using VLC Player to debug the issue.
The errors when opening two cameras simultaneously using c++ opencv ( the code is similar to https://code.ros.org/trac/opencv/browser/trunk/opencv/samples/cpp/starter_video.cpp?rev=4705 )
e557822#e557822-T740:~/Desktop/Camera/starter_video2$ ls /dev/video*
/dev/video0 /dev/video1 /dev/video2
e557822#e557822-T740:~/Desktop/Camera/starter_video2$ ./starter_video2 0 1
VIDIOC_QUERYMENU: Invalid argument
VIDIOC_QUERYMENU: Invalid argument
VIDIOC_QUERYMENU: Invalid argument
VIDIOC_QUERYMENU: Invalid argument
VIDIOC_QUERYMENU: Invalid argument
VIDIOC_QUERYMENU: Invalid argument
VIDIOC_QUERYMENU: Invalid argument
press space to save a picture. q or esc to quit
init done
opengl support available
libv4l2: error turning on stream: No space left on device
VIDIOC_STREAMON: No space left on device
UPDATE 7/24 :
This Ubuntu 12.04 (32- bit) is a guest OS running on VMware Fusion. The host OS here is OSX 10.9.4 running on a Mac Pro computer hardware.
$ lsusb -t
/: Bus 02.Port 1: Dev 1, Class=root_hub, Driver=uhci_hcd/2p, 12M
|__ Port 1: Dev 2, If 0, Class=HID, Driver=usbhid, 12M
|__ Port 2: Dev 3, If 0, Class=hub, Driver=hub/7p, 12M
|__ Port 1: Dev 4, If 0, Class='bInterfaceClass 0xe0 not yet handled', Driver=btusb, 12M
|__ Port 1: Dev 4, If 1, Class='bInterfaceClass 0xe0 not yet handled', Driver=btusb, 12M
/: Bus 01.Port 1: Dev 1, Class=root_hub, Driver=ehci_hcd/6p, 480M
|__ Port 1: Dev 6, If 0, Class='bInterfaceClass 0x0e not yet handled', Driver=uvcvideo, 480M
|__ Port 1: Dev 6, If 1, Class='bInterfaceClass 0x0e not yet handled', Driver=uvcvideo, 480M
|__ Port 2: Dev 7, If 0, Class='bInterfaceClass 0x0e not yet handled', Driver=uvcvideo, 480M
|__ Port 2: Dev 7, If 1, Class='bInterfaceClass 0x0e not yet handled', Driver=uvcvideo, 480M
$

Answer 1

This is USB bandwidth problem, not VLC one.
VIDIOC_STREAMON: No space left on device is message which is given when USB bandwidth is full.
Most modern USB cameras do high speed, high density image output. USB2 is limited to 480Mbps which is 60 megabytes/s theoretical. In practice all kind of overhead will use about half of it and 30 megabytes/s is maximum you can get. This means camera can send 1 megabyte image at 30fps.
All you can do is get motherboard with multiple USB buses. All small computers tend to have just one. High-end gaming motherboards have 2..3 USB2.0 buses. You can see it under linux:
$ lsusb -t
/: Bus 04.Port 1: Dev 1, Class=root_hub, Driver=xhci_hcd/4p, 5000M
/: Bus 03.Port 1: Dev 1, Class=root_hub, Driver=xhci_hcd/4p, 480M
|__ Port 4: Dev 2, If 0, Class=scard, Driver=usbfs, 12M
/: Bus 02.Port 1: Dev 1, Class=root_hub, Driver=ehci_hcd/2p, 480M
|__ Port 1: Dev 2, If 0, Class=hub, Driver=hub/8p, 480M
|__ Port 5: Dev 3, If 0, Class='bInterfaceClass 0xe0 not yet handled', Driver=btusb, 12M
|__ Port 5: Dev 3, If 1, Class='bInterfaceClass 0xe0 not yet handled', Driver=btusb, 12M
/: Bus 01.Port 1: Dev 1, Class=root_hub, Driver=ehci_hcd/2p, 480M
|__ Port 1: Dev 2, If 0, Class=hub, Driver=hub/6p, 480M
|__ Port 3: Dev 3, If 0, Class=vend., Driver=rts5139, 480M
|__ Port 5: Dev 4, If 0, Class='bInterfaceClass 0x0e not yet handled', Driver=uvcvideo, 480M
|__ Port 5: Dev 4, If 1, Class='bInterfaceClass 0x0e not yet handled', Driver=uvcvideo, 480M
$
Most probably both cameras are connected to same bus.
Other option is to reduct FPS speed or resolution. But this not always helping because I have seen cameras which reserve 80% of USB bandwidth anyway.