You're basically talking about the shift system. A works for 20hrs a week, B works for 20hrs a week. A spends more time with spouse(A), who does the same at their workplace, and B spends more time with spouse(B), who does the same at their workplace. Sounds great.
But, it falls apart to the same logic GP proposed, that the reason you have dual income households is that they are richer than single income ones. Households where people both work 40hrs = 80hrs will be ahead of those that work only 40hrs total. So everyone will descend to working 80hrs too.
Of course, taking mine and GPs logic to it's conclusion is silly - people will have a point where they stop comparing with others and tradeoff less money for less hours. But looking at reality, it seems like that limit is very high! And it only happens at an already very high salary. A 40hrs/week SWE might not go to a high finance 70hrs/week job, because they're already comfortably paid. However these two are top 1% jobs in the world, and the quality of life is probably not too different. But if you go down to the lower rungs, people are more inclined to compare themselves with peers and tradeoff double hours for the next rung, which entails a much better quality of life (as a % increase)
