I think the idea is with 2 incomes, both partners need to remain employed in order to keep up, since people's expenses usually closely match their income. So if 1 of the 2 loses their job, that 1 of the 2 needs to find new employment to maintain their lives.

In the scenario of 1 working partner, if that 1 person loses their job, you can have 2 people looking for jobs in order to get back to normal.