Modelling a xml file

[wj210]

Hi, i am new to mujoco and have no idea how to alter a default environment in mujoco.
Say, if i want to modfiy the doubleinvertedpendulum.xml file, to include a second cart with poles body. How would i go about doing it?
I tried the naive approach of copying the body tags and pasting it in within worldbody and gotten no success from it. I experimented
with the offset positions as well but doesn't to be able to simulate it using the simulate command in mujoco bin folder.