This file is a urdf which mimics a parallel linkage.

mimic_parallel_link.urdf

<?xml version="1.0"?>
<robot name="parallel_link">
  <link name="base_link">
    <visual>
      <origin xyz="0 0.25 0" rpy="1.57 0 0" />
      <geometry>
        <cylinder radius="0.02" length="0.5" />
      </geometry>
      <material name="blue">
        <color rgba="0 0 .8 1" />
      </material>
    </visual>
  </link>

  <joint name="joint0" type="revolute">
    <parent link="base_link"/>
    <child link="link1"/>
    <origin xyz="0 0 0" rpy="0 0 0" />
    <axis xyz="1 0 0" />
    <limit lower="-1.57" upper="1.57" effort="0.5293" velocity="6.159937255" />
  </joint>

  <link name="link1">
    <visual>
      <origin xyz="0 0 0.25" rpy="0 0 0" />
      <geometry>
        <cylinder radius="0.02" length="0.5" />
      </geometry>
      <material name="yellow">
        <color rgba=".8 0 .8 1" />
      </material>
    </visual>
  </link>

  <joint name="joint1" type="revolute">
    <parent link="base_link"/>
    <child link="link2"/>
    <origin xyz="0 0.5 0" rpy="0 0 0" />
    <axis xyz="1 0 0" />
    <limit lower="-1.57" upper="1.57" effort="0.5293" velocity="6.159937255" />
    <mimic joint="joint0" multiplier="1" offset="0" />
  </joint>

  <link name="link2">
    <visual>
      <origin xyz="0 0 0.25" rpy="0 0 0" />
      <geometry>
        <cylinder radius="0.02" length="0.5" />
      </geometry>
      <material name="green">
        <color rgba="0 .8 0 1" />
      </material>
    </visual>
  </link>

  <joint name="joint2" type="revolute">
    <parent link="link1"/>
    <child link="link3"/>
    <origin xyz="0 0 0.5" rpy="0 0 0" />
    <axis xyz="1 0 0" />
    <limit lower="-1.57" upper="1.57" effort="0.5293" velocity="6.159937255" />
    <mimic joint="joint0" multiplier="-1" offset="0" />
  </joint>

  <link name="link3">
    <visual>
      <origin xyz="0 0.25 0" rpy="1.57 0 0" />
      <geometry>
        <cylinder radius="0.02" length="0.5" />
      </geometry>
      <material name="red">
        <color rgba=".8 0 0 1" />
      </material>
    </visual>
  </link>

</robot>

Your example is not use in gazebo
if you want to to solve the issue that URDF not support Closed loop chains.
you can reffer to:https://github.com/wojiaojiao/pegasus_gazebo_plugins or http://wiki.ros.org/Angel_jj/closed_loop_plugin