Potentially a two income household allows it's expenses to grow to take advantage of both incomes to some extent.

As a couple with two high incomes you actually have to try really hard not to let expenses get to the point where you can't live without both incomes. So in effect you now have 1 big income that you rely on.

In terms of the fragility, there is more chance of losing 1 out of 2 jobs (i.e. 50% of the income) than there is of losing 1 out of 1 jobs (i.e. 100% of the income). (I hope I explained this right - but it's how my thinking goes).