how can I use two model in rviz ?

edit

I use two laser sensor(sick_tim551 2050001) in my robot.

when I use two laser sensor in my robot it seems to work as one.

Also, In rviz, there is only one sensor model, and operate as one.

How can I use two sensor model in rviz and operate separately each sensor.

plz help. :(

oh. I'm sorry. Here is my launch file.

<group ns="lidar1">
         <param name="robot_description" command="$(find xacro)/xacro --inorder '$(find sick_tim)/urdf/example.urdf.xacro'" />
         <node name="robot_state_publisher" pkg="robot_state_publisher" type="state_publisher" />
         <node name="sick_tim551_2050001" pkg="sick_tim" type="sick_tim551_2050001" respawn="false" output="screen">
         <param name="hostname" type="string" value="172.16.1.209" />
         <param name="port" type="string" value="2111" />
         <param name="timelimit" type="int" value="5" />
  </node>
</group>

<launch>

<group ns="lidar2">
         <param name="robot_description" command="$(find xacro)/xacro --inorder '$(find sick_tim)/urdf/example.urdf.xacro'" />
         <node name="robot_state_publisher" pkg="robot_state_publisher" type="state_publisher" />
         <node name="sick_tim551_2050001" pkg="sick_tim" type="sick_tim551_2050001" respawn="false" output="screen">
         <param name="hostname" type="string" value="169.254.12.60" />
         <param name="port" type="string" value="2112" />
         <param name="timelimit" type="int" value="5" />
     </node>
<group>

</launch>

I seperate two sensors with group ns . Also I saw ,using rqt_graph, each sick_tim551_2050001 node share just one TF Node.