Was trying to figure this out regarding a form 4868 in April, 2023. From the instructions on form 4868:
If you and your spouse each filed a separate Form 4868 but later
file a joint return for 2022, enter the total paid with both Forms 4868
on the appropriate line of your joint return.
If you and your spouse jointly file Form 4868 but later file separate
returns for 2022, 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.
