Wages, in this context, are what you expect to appear in box 1 of your W-2.The IRS recommends that the additional withholding be applied to the higher-paid spouse and that the lesser-paid spouse should simply claim zero withholding allowances, as this is usually more accurate (due to the way that withholding is actually calculated by payroll programs, you may wind up with less withheld than you want if you split it).