since the spouse may have different types of income subject to different tax rates another way (often available in software used by professionals) is to calculate taxes as if MFS. then the joint tax is apportioned in relation to the MFS tax (before reduction for withholding and estimated tax payments). Then from the apportion amount each taxpayer's withholding and estimated tax payments are subtracted. Issues do arise if the taxpayers itemize or if there are tax credits. the parties would have to agree on how they are to be allocated.
