In progressive tax situations that is called the 'marriage penalty' (second salary is all taxed at the higher marginal rate) which is mitigated somewhat by changing the tax thresholds for married people filing jointly. You can file separately of course, and it seems a number of people are living together and filing as individuals, it gets complicated as you get kids because they become dependents (and a tax credit) on one of the people's return.

There is a reason all this stuff keeps a legion of tax attorneys and CPAs employed.