stereo with two webcams

edit

hi all, i have two webcams and want run these at same time. i use usb_cam package and mylaunch is this:

<launch>
 <group ns="camera1">
  <node name="usb_cam1" pkg="usb_cam" type="usb_cam_node" output="screen" >
    <param name="video_device" value="/dev/video1" />
    <param name="image_width" value="640" />
    <param name="image_height" value="480" />
    <param name="pixel_format" value="mjpeg" />
    <param name="camera_frame_id" value="yuyv" />
    <param name="io_method" value="mmap"/>
  </node>
  <node name="image_view" pkg="image_view" type="image_view" respawn="false" output="screen">
    <remap from="image" to="/camera1/usb_cam1/image_raw"/>
    <param name="autosize" value="true" />
  </node>
 </group>

<group ns="camera2">
  <node name="usb_cam2" pkg="usb_cam" type="usb_cam_node" output="screen" >
    <param name="video_device" value="/dev/video2" />
    <param name="image_width" value="640" />
    <param name="image_height" value="480" />
    <param name="pixel_format" value="mjpeg" />
    <param name="camera_frame_id" value="yuyv" />
    <param name="io_method" value="mmap"/>
  </node>
  <node name="image_view" pkg="image_view" type="image_view" respawn="false" output="screen">
    <remap from="image" to="/camera2/usb_cam2/image_raw"/>
    <param name="autosize" value="true" />
  </node>
 </group>
</launch>

but when roslaunch my launch file, one webcam run and others is off and instead that the webcam from laptop is on! what is problem?! please help me