URDF with two differently prefixed UR robots

example_scene.urdf.xacro

<?xml version="1.0"?>
<robot xmlns:xacro="http://www.ros.org/wiki/xacro"
       name="example_scene" >

  <!-- robot arms -->    
  <xacro:ur5_robot prefix="a_bot_" joint_limited="false"
    shoulder_pan_lower_limit="${-pi}" shoulder_pan_upper_limit="${pi}"
    shoulder_lift_lower_limit="${-pi}" shoulder_lift_upper_limit="${pi}"
    elbow_joint_lower_limit="0.0" elbow_joint_upper_limit="${pi}"
    wrist_1_lower_limit="${-pi}" wrist_1_upper_limit="${pi}"
    wrist_2_lower_limit="${-pi}" wrist_2_upper_limit="${pi}"
    wrist_3_lower_limit="${-pi}" wrist_3_upper_limit="${pi}"/>
  
  <xacro:ur5_robot prefix="b_bot_" joint_limited="false"
    shoulder_pan_lower_limit="${-pi}" shoulder_pan_upper_limit="${pi}"
    shoulder_lift_lower_limit="${-pi}" shoulder_lift_upper_limit="${pi}"
    elbow_joint_lower_limit="0.0" elbow_joint_upper_limit="${pi}"
    wrist_1_lower_limit="${-pi}" wrist_1_upper_limit="${pi}"
    wrist_2_lower_limit="${-pi}" wrist_2_upper_limit="${pi}"
    wrist_3_lower_limit="${-pi}" wrist_3_upper_limit="${pi}"/>

  <joint name="world_a_bot_joint" type="fixed">
    <parent link="world" />
    <child link="a_bot_base_link" />
    <origin xyz="0 -.5 0" rpy="0.0 0.0 0.0" />
  </joint>

  <joint name="world_b_bot_joint" type="fixed">
    <parent link="o2as_ground" />
    <child link = "b_bot_base_link" />
    <origin xyz="0 .5 0" rpy="0.0 0.0 0.0" /> rpy="0.0 0.0 0.0" />
  </joint>
</robot>