That somebody driving a truck or car in the normal way is doing more work than a person supervising a self driving car is the whole point of self-driving cars. They take all of the activity out of driving, but none of the tedium.
I don't think it's realistic to expect people to function well under those conditions.
When some people learn to drive they can take courses with instructors who sit in the passenger seat and have a brake pedal. When the student driver runs over and kills a pedestrian while the instructor is on their phone, is it the instructor or the student who should face liability? Why not both?
In England certainly the person supervising a learner driver is subject to the same laws about alcohol, mobile phone use, etc as someone actually driving- and can be convicted on a charge of aiding and abetting any crime the learner driver commits. This is the case regardless of whether they are a professional driving instructor with their own brake pedal, or a parent supervising their child driving the family car.
In Portugal there is an interesting law regarding this: The instructor/examiner is always liable for an accident during driving lessons/exams, except if the accident resulted from an action where the learner disobeyed a direct order by the instructor. In that case the learner takes responsibility.
I don't think it's realistic to expect people to function well under those conditions.