some work is not about volume but about quality. and exactly the fact that the work can not be measured is the problem. some work can not be done effectively when you are tired, and some work is even dangerous. driving for example. paying attention to safety and security requires you to be a wake and able to focus.
as a programmer i have good days and bad days, and noone can say whether my work is the best i can manage or is slacking because i have a second job. and if i am spending more than my market rate salary as a programmer affords then i need to change my lifestyle, not get a second job.
in germany for example the laws for the maximum time of work an employee is allowed to do in a day covers all jobs. two full time jobs would simply not be possible because you are required to have 11 hours of break between two workdays.