I thought the Teamsters were more into the loading and unloading, and the drivers were often owner/operators.
Never heard of more than one person operating a truck at a time.
Laws define how long a driver can drive between mandatory breaks - basically making sure they get their sleep. Truckers keep log books that are legally required to be accurate and will be inspected if they get stopped by police. Falsifying logs is a criminal offense.
Some folks do what's called "team driving" where two people in one truck take "shifts" and drive non-stop (this with a sleeper in the cab). Often it's husband/wife teams. I've honestly considered doing it with my wife after the kids are out of the house and everything's paid off. Get paid to see the country with my wife - pretty cool, actually. Hemorrhoids, not so much.