OK, first thing is open a single car scene of your choice, lets say MENCS19 mod Camaro scene. set your views to the 4 quad, one having to be the top view where you will work, next is to freeze the floor, then move the lights away for more room, then select the entire model, holding down the shift key, pick the move icon on top ribbon, then move this down or up, left or right, then let the shif key up, a box will pop up asking to name the new model, I usually just say ok for their suggestion, now you have 2 cars in the scene. Now open the material editor, look at the balls and find one that isn't used, then pick the Camaro ball and move it to that location, in the name bar you must rename it, I usually just add a number after the word so it reads Camaro2, now add in your paint for the first car. Now add in the paint for the second car. I use the eye dropper to pick the first car, but not the second. After adding paint for second car, just drag that ball to the second car in the scene, paying close attention to making sure all the body parts are covered. Some do with the first drag, some don't depends on how the original scene was set up.
Here are screen shot of how I do it. I'm sure there are other ways.
Thanks to
NokiaCupSeries for the flats used in this post.