Its not difficult.
The DOT leaves that up to individual companies.
Teamsters has a page on this.
PDF Version 49 CFR Parts 380 and 391 Applicability and Compliance: The Federal Motor Carrier Safety Administration established standards for mandatory traini
With that said, there is or was a experiment to do Aussie Style Road Trains in the Dakotas, up to 5 semi trailers and 600,000 pounds.
Newbies do not get to pull LCV's This is a old hands game. Personally I carefully chose to game the system so I never had to worry about doing that. Essentially by never getting the endorsements in the then new CDL days on purpose among other things. It was a good move at that time because many LTL companies failed in my area and never came back because our yards were just too small. Sold off for the more profitable zoning land.
In the end the Insurance Companies and trucking companies agree who to use as trainers and who to stick into the LCV. There is no real teeth in DOT FMCSA etc. In short the industry regulates its own training on the job, carefully choosing those who have experience and excluding newbies.