Adding two robots in gazebo tfs problem and urdf files

I want to add two robots in gazebo. The first robot is a quadrotor from hector package the second robot is husky I used apt-get to download both of then except the husky_description. I used git clone "link" I changed in the mesh file of the husky. My launch file is the following: The husky_spwan.launch file is the following: freq: 10.0 sensor_timeout: 1.0 publish_tf: true odom_used: true vo_used: true output_frame: /world the box.launch file only launch the gazebo client and server. first I faced a problem becasue both robots have two links with the smae names. Therefore, I changed in the urdf file of the husky from base_footprint to husky_foot_print and from base_link into husky_base link. ( I dont know if what I did it write or wrong) Anyway, when I run the quad rotor alone and I check the tf I find all the tfs and links and joints that are specified in the urdf file for the quad rotor. Also, when I run the husky alone I find all its links and joints. but when I run them together as the shown in the launch file above the links that their parent is the "base_link" in the quadrotor disapear from the tf tree. Also, I dont know what is the fixed frame for the husky or from where to spacey it ?? if I want it to be the world frame as for the quad rotor what should I do ?? Also, there is a node that is called robot_state_publisher >> should I call it twice for each robot or once for both ?? because it exist in the launch file of the hector_quadrotor and also you can see it in the launch file for the husky ?? Actually I could not run if the two robot_state_publisher nodes so I had the following: This node I added it in the launch file of husky: this node exist in the launch file of the quadrotor: