It is important that there be sufficient overlap. It is less important that there be complete overlap. There is a downside to my wife working while I am free, but an up-side too if we are better able to distribute tasks like childcare between us.

Maintaining sufficient overlap is partly a matter of individual schedule preferences, and it gets easier as amount of free time increases (though, obviously, at a cost of overlap in work time) and as scheduling flexibility generally increases. Currently, if my wife has a "9-5" and I am stuck working something with weekend shifts, my wife typically lacks the flexibility to shift her schedule to match mine. What I am most strongly arguing for is increased flexibility where it is feasible.