Aggregate multiple topics of same name and type

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First, you should see #q275079 answer for a better understanding of what robot_state_publisher and joint_state_publisher nodes really do.

Regarding your question I'm not sure to completly understand it : You can already get the pose of each robots using TFs by listenning to the transform between map and robot_N/base_link (assumming that the frame have a prefix like robot_N if you spawn your robots propperly) with the command rosrun tf tf_echo source_frame target_frame.

Imagine all the robots publish their pose and name on the same topic. There is a node subscribing to that topic and would be able to then publish a topic that has all the robot positions that it has recieved.

You could create a node listenning to the transform of each robots and use an array of geometry_msgs::Poseto publish them.