Not sure how to apply a legal framework, but in my area this is often enforced by the lack of daycare options for <2yos (the threshold might be lower, but about that much).
Very rare to see parents around me take their parental leave at the same time bc they'd be risking still being on a waitlist when they're done.
As for how you end up with just those two, I think the answer is divorce: you don't spend the energy to maintain your relationship with your spouse. Before you had kids you could invest your time and energy in your spouse and your career, then the kids come along and your relationship with your spouse weakens, either all the way to divorce or to just a degraded state.
Does that solve the problem? Now you're trying to prioritise kids and career while also being a single parent for half the week.
This situation is in development for me so we will see. 0 and 3 year old
And I do wonder if this is about the engineer mindset ('I've reduced this to a previously solved problem, I just let the subroutine handle it') or about the way that men are socialized in my culture (just take her out to dinner twice a year- Mother's Day and Valentine's- and remember her birthday and you're meeting social expectations) or what. But it's the biggest threat to my (and I'd hazard a guess most other) marriages.