I think that this can be achieved with a softer approach. Imagine that when you sign up for a job, you select how many days per week do you want to work, from 3 to 6, with proportionate wage scale. That you can select what hours do you want to work. That it's easy to change this arrangement while you're already working.
Now, why is this fantasy not a reality? Not because of laws; laws allow this. But:
1) These creative arrangements require non-standard agreements, and a lot of additional bureaucracy, because in a lot of countries, all the bureaucracy machinery allows it, but is really not optimized for use-cases like that.
This can be solved through careful policy work, removing necessary paperwork, streamlining processes, etc.
2) Economies of scale. Having a full-time 40h/week employee that gets X money as salary is cheaper than having 2 half-time 20h/week employees that get X/2 money because of management overhead, cost of their office spaces and other stuff like that.
This would be solved if 20h/week employees would understand this and get X/2-Y money. Also, a lot of these scale issues are being solved by modern world anyway, because we're learning to telecommute, work together in more effective ways, automate management tasks, etc.
3) Culture.
And I think this is the most important one. It's not in the laws, it's in the people's heads. We need to convince people that one person wants to dedicate work 60 hours per week, spend free time that's left on professional education and succeed in his career, and another is quite OK to work for 20 hours per week, get less promotions, learn less new stuff and earn less money, and spend all his free time with his kids. We need to stop labeling the first of these persons as "successful" and second one as a "failure": it's OK to be both of them. And they need to be able to work in the same office, on the same project, fully understanding difference in each other's views and being mutually acceptful.
I don't know how to do that, but it seems that this cultural change is already slowly happening. Still, advancing it would bring the real change much faster and more effectively than writing new laws.