From the Tax Policy Institute using 2015 household incomes:

Bottom fifth, $22,800 or less
Next fifth, $22,801 to $43,511
Middle fifth, $43,512 to $72,001
Second from top fifth, $72,002 to $112,262
Top fifth, $112,263 or more.

So crudely splitting the fifths and remembering this is household income, so it includes two, three, or even four salaries, we get
 
Bottom third (poor), below about $33,000
Middle income, $33,000 to $92,000
Rich, above $92,000.

So a two income family earning $40,000 each is top of the middle class, almost rich. But I bet they don't feel like it.

More importantly that two income family is in the top 40% of households, and according to CNBC a few years ago:

If you count earned income tax credits it's even worse. The top 40 percent of wage earners in America pay 106 percent of the taxes. The bottom 40 percent pay negative 9 percent.