We have exactly this problem in the UK.

A couple each home earns x, each gets taxed on x. Each gets the tax free allowance on the fist £12.5k of the annual income. Each gets the full basic rate slice before they hit higher bands etc.

If one of the couple earns 2x and the other zero, then only one can use their tax free allowance and they get one slice of the basic rate band etc.

They still have the same pre-tax income for the same household.

Personally I think people should be allowed to opt in to sharing taxable income.