Ideally, work days should just be two days for each person, and maybe with some people choosing three days to cover the weekend, if it's that type of job that people would need someone to attend to them anyday of the week. A two or three days typical work week would even cut out unemployment for many, because after your work week ends, someone else's would begin.
Why this wouldn't hold : In a capitalist society, people want to get the most profit, paying twice for one job position isn't appealing to most employers. Other than that, it's a great idea. Reduces stress, boredom, poor health, unemployment and sickness, increases: Good mental health, hobbies, opportunities for more financial freedom, rest.