This obsession with hours might be the root of the problem.
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.