There is more to it than just the endorsement on your CDL. I am sure somewhere early in your career your company put you thru some sort of orientation and training program to pull double or triple trailers. It never comes up on the roadside but will come up if there is ever a DOT audit or serious accident. Just another piece of paper in the file that shows you were instructed how to safely operate this type of equipment, similar to the new hire orientation training required of drivers with less than 1 year of experience.
I was pulling doubles back in 1987, no special training or certificate for it. Grandfathered into the CDL when it was instituted, doubles/triples endorsement was automatic since I’d held a chauffeurs license, the predecessor to the CDL & had doubles experience. Have heard of the newer LCV requirement, but from what I’ve read/heard it was intended for pup pullers not tractor/jeep/trailer/stinger-booste combinations.
Could be wrong, but I’ve never been asked for it with such a setup.