How? Taxes for couples are generally just doubled. You can actually save on taxes by not being married and maneuvering who claims what (house, dependants, etc).
Taxes are generally Progressive actually so the overall rate is higher for one person then two splitting.
Some major tax deductions like the mortgage deduction are per person as well, so two people with a shared income get twice the deduction as one person with an income
"Some major tax deductions like the mortgage deduction are per person as well"
This doesn't sound right.
"so the overall rate is higher for one person then two splitting."
That depends if that one person is the higher or lower earner. This statement really only works if you have two unequal salaries. Since it's progressive, you're only paying the higher taxes on the amount that is over the threshold.
I don't know what to tell you. The code is pretty straightforward. You can deduct the mortgage interest on 375 k per person. You get twice the tax deduction with two people as one.
Regarding the general taxes you are correct in that it only helps if you have two people in different tax brackets. This is extremely common
Some major tax deductions like the mortgage deduction are per person as well, so two people with a shared income get twice the deduction as one person with an income