JUST ABOUT everybody agrees that the income tax is dreadfully unfair to those marriages in which both partners have jobs. Just about everybody is wrong: the unfairness is modest, at best -- but that's irrelevant. President Reagan and Congress, responding to the firm opinion of just about everybody, are about to fix the law. They are going to fix it by making it a good deal less fair to all the people who aren't married couples with two incomes.

The sad truth is that there's no possible way to make a progressive income-tax system equally fair to all three classes of taxpayers -- single people, married couples with only one income, and married couples with two income. The Treasury Department and Congress know that perfectly well. But they also know that when they change the law and relocate the inequity, it takes about 10 years for the new victims to recognize the outrage and crank up a political campaign to correct it. The remedy to the marriage penalty is a prominent feature of the tax bill now before Congress. The remedy to the remedy, shifting the inequite to someone else, will be a prominent feature to the Tax Reform Act of 1991.

Twelve years ago single people complained bitterly that they were taxed at much higher rates than married couples. Congress responded by lowering the the tax on single people. As a result, over the past decade, two-income couples have begun bitterly complaining that they are taxed more heavily than if they were single. That's true. A couple with $40,000 a year pays more taxes than two single people with $20,000 a year each. President Reagan's tax bill proposes to respond by giving a two-income family a tax deduction for part of the smaller income. It is to be 5 percent in 1982, and 10 percent subsequently.

That will presumably placate the two-income couples. But what about everybody else? It will mean that a one-income family with $40,000 a year will pay significantly higher taxes than a two-income family with $40,000 a year. What's fair about that? And if married couples want to be taxed as though their incomes were separate, why shouldn't they be taxed at the higher rates for single people?

There's worse. Under the Reagan bill, the second-income deduction would be allowed whether the couple itemizes its other deductions or not. Here comes the super-deduction. Dozens of religious and charitable lobbies have been working and hoping for years to establish the super-deduction. If Congress establishes the precent of one super-deduction, hundreds of decent and respectable causes will demand equal treatment. They urgently want to offer their supporters the same status for contributions -- that they will be deductible even for people who don't itemize, or whose deductions are less than the standard minimum. But why should some deductions have super-status, when others do not? As a question of equity, that's too much to inflict on the Congress that will have to write the Tax Reform Act of 1991.