Have all W-4 and W-4P forms set to SINGLE as the filing status and leave the rest of the form blank.
If that is not enough simply figure out how much more is needed and divide that by the number of payments you get on one of those things then add that amount to line 4C of one of those W-4 forms.
Even if you file jointly you can choose the SINGLE option on the NEW W-4 form ... that is allowed and legal. On the old W-4 form there was actually an option called "Married but withhold at the higher single rate" which was removed in the newly designed form.
