A husband and wife currently file jointly.  In 2019, the wife started a job classified as a "statutory employee" and no federal income tax is withheld from her pay, therefore, they would like to make quarterly estimated income tax payments in 2019.  Questions are:

1. If they use the payment options on the IRS web site (Direct Pay or Pay By Credit/Debit Card), would the estimated tax payments be entered using only the wife's social security number (since it relates to her job income)?  

2. If it asks for both, should the wife's be listed first, then the husband's (the tax return shows the opposite - the husband's first, then the wife's)?  

3. If both are listed and then the couple decide to file separately for 2019, how will these estimated payments get credited to the wife if both social security numbers are used (or if the husband's social shown first/primary)?

Thanks in advance for any replies.