Might just be me but I'm missing the relevance. The point is that a household which has already maxed out its possible number of workers no longer has the option of sending someone else to work to deal with a sudden expense, so a two worker household is more fragile than a single worker household at the same income. The only reason I bring up productivity is to mention that it hasn't come down in proportion, which this graph doesn't show either.
https://www.financialsense.com/sites/default/files/users/u61...
http://www.pewresearch.org/ft_dual-income-households-1960-20...
If you want to zoom in on more recent events wikipedia has a pretty compelling chart here: https://en.wikipedia.org/wiki/Household_income_in_the_United...