Correct way to format URDF for 6 DOF between base_link and robot_arm_base_link using JointTrajectoryController

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Hello Gazebo Users,

What is the proper way to write a URDF in order to create a base_link that has 6 DOF and then attach a pair of robot arms to the underside? I would be doing this to create a walking robot. I would like to have the 6 DOF together under one controller using a JointTrajectoryController, as well one controller for each robot arm with a JointTrajectoryController.

I have tried so far creating a base_link using a cube, then connecting 6 other very small links with the 6 DOF required (3 prismatic and 3 revolute). I then tried connecting the base link of the robot arm to the 3rd revolute link. However, when I did this, the robot arm fell through the base_link (cube) and landed on the ground.

Any help would be appreciated.