Sure and under income taxes, Couple B probably pays much less since US income tax structure is gives massive benefits to couples with single income. It may not be enough to offset joint income. As with most economic changes, there is massive web of things.

> US income tax structure is gives massive benefits to couples with single income

It gives massive structural advantages to couples with low income, in the form of a lower marginal tax rate. Does it really discriminate between single and dual income though? I wasn't aware of that.