How to spawn one robot onto another in gazebo?

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Hello,

This is the copy-paste version of this question: LINK

I have two different and independent robots. (TWO different URDF files)

In gazebo i want to spawn+attach the second robot to the top of the first robot (the top is defined by the eef_link of robot1).

Of course, in this way, i want these robots to be connected, i.e., if robot1 actuates/moves its eef then since robot2 is attached to it, therefore robot2 needs to move with robot1's eef.

Can you please provide a launch file that performs this spawning mechanism in gazebo?

Thank you in advance.