How to simulate multi-robots transportation in gazebo?

edit

Here is the code that worked for me, and as steve said on the comment above, you have to pay attention for hardcoded topics. For namespace:

<arg name="model" default="$(find your_package)/urdf/robot.urdf.xacro"/>

<!-- Launch gazebo -->
<include file="$(find your_package)/launch/gazebo.launch">
  <arg name="model" value="$(arg model)"/>
 </include>  

<group ns="robot1">
        <param name="tf_prefix" value="robot1_tf" />

        <include file="$(find your_package)/launch/robot.launch" >
          <arg name="init_pose" value="-x 0 -y 1 -z 0" />
          <arg name="robot_name"  value="robot1" />
        </include>

        <include file="$(find ira_laser_tools)/launch/laserscan_multi_merger.launch">
          <arg name="robot_name" value="robot1"/>
        </include>
   </group>

then for robot.launch:

<launch>
    <arg name="robot_name" default="your_default_value"/>
    <arg name="init_pose" default="your_default_value"/>


<node name="urdf_spawner" pkg="gazebo_ros" type="spawn_model"
     args="$(arg init_pose) -urdf -param /robot_description -model $(arg robot_name)"
     respawn="false" output="screen" />

    <node pkg="robot_state_publisher" type="robot_state_publisher" 
          name="robot_state_publisher" output="screen"/>

</launch>

You can spawn multiple robot instances in gazebo by launching the xacro in different namespaces (and hopefully starting positions so they don't explode) as long as you don't hardcode any topics in the global namespace in your software. This would make it so a command to one would be sent to all. If you use a local namespace for all this should just work out of the box, although slow