Hello,
I had a mod called Additional Customization that was available in Steam Workshop that allowed me to buy trailers that we can't buy in the vanilla game (including doubles), like trailers with cars, pipes, other trucks, etc. But unfortunately this mod is not available anymore and now I don't know if there's another way to do it, because I see many people with trailers like that in multiplayer. Hope someone can help me and sry for bad English, I'm Brazilian.