See IRS Form 4868 Application for Automatic Extension of Time
To File U.S. Individual Income Tax Return instructions on page 2 - https://www.irs.gov/pub/irs-pdf/f4868.pdf
If you and your spouse jointly file Form 4868 but later file
separate returns for 2015, you can enter the total amount paid with
Form 4868 on either of your separate returns. Or you and your
spouse can divide the payment in any agreed amounts.
