i tell ya how it works: first you need to be lvl 7 or 8 and you need to have yr own truck(you will see when you take trailers on jobs, a yellow/white trailer will be apear and attatched to your truck at evry job.
when you have the yellow/white trailer behind yr truck, you can go to the service point and change your chassis to tandem chassis.
go to upgrade cabin stuff(accessoires) and you will see the option kuzov behind yr cabin, now click on it to pick one of the containers that you want behind yr cabin.
have fun driving you can pick only 1 kind of trailers at companys(yellow/white)