Most businesses have given up trying to measure produced value per worker (and certainly don't reward based on value produced). The 40 hour work week just means that it is expected that you work 40 hours a week, 48 weeks a year (give or take, depending on vacation practice/laws) -- in order for the employer to pay you "full benefits". That typically includes ideas about a living wage (not in all parts of the world), and some form of social safety net, such as health care, insurance etc.
A 20 hour work week, would just mean that the expected time most need to put in, to get the basic salary/coverage would be cut in half.
I don't think a smooth, nor easy transition is possible -- you can look at like doubling hourly wages, or halving all prices, or through some other lens: but I think that we are getting accustomed to thinking of hourly pay, hourly wages/compensation etc is part of the problem.
The idea of a work week: that one can work "full time" for one employer doesn't really say that much about how much time one puts in -- it's more about what responsibility that employer have for the employee. And the "norm" of 40 hours is used as a guide to compensate extra for overtime, or to reduce compensation for people that can't or won't work "full time".
Maybe the best way forward, if one believes that some form of "fair" distribution is possible under capitalism, is to have all businesses be 100% employee owned. It's not entirely clear why anyone else should benefit directly from being able to pay less for work put in by workers, and pocket the difference. Especially considering that a lot of the capital invested might be borrowed money -- which under fractional reserve banking is just money made up out of thin air and given to those that need it the least.
Make health insurance not tied to employment. (It could be universal coverage handled by the government, it could be what we're half trying where everyone buys their own. As long as it's not thru the employer, it doesn't matter for this.)
Make things like FMLA leave not depend on working enough hours.
Remove the caps that some payroll taxes have.
Some per-employee costs are unavoidable. If my team has 10 people instead of 5 people, and they're all available at different times, that's a cost. The work has to be divided differently, I have to remember more interpersonal crap, there will be communication delays, etc.
But as much as possible should either be handled by Someone Else or made dependent linearly on hours or pay, rather than being tied to employment as a boolean or to a threshold number of hours or amount of pay.
I have friends in France who sneak into their academic research labs after hours because of this exact reason (well, not 20 hours, but set max work hours)... seems super silly to me.
We're talking about an unusual case, though. You have to realize the most people hate their jobs. They're not following their passions. They find work tedious, uncomfortable, and frequently demeaning. For these people, the bulk of humanity, a cap on working hours frees them to do something they find much more interesting.