controlling two car models separately and ROS melodic - issues.

[alexantosh]

hello. am using ros melodic. also build using catkin build.
unfortunately only the world_test.launch works for me. The car shows up and can be controlled.
For the rest, the world loads but the car models dont load.

Anyway, most importantly, i somehow managed to create a second car model from the launch file by following the tutorial below.

https://www.theconstructsim.com/ros-qa-130-how-to-launch-multiple-robots-in-gazebo-simulator/

what i want to know is how I can independently control this second car. thanks.