When a joint return is filed and only one spouse owes a past due amount, such as child support, an education loan, or prior year taxes, the spouse who is not obligated for the debt can be considered an "injured spouse." The couple should have filed Form 8379, Injured Spouse Allocation, with their original return. This form can be e-filed. If they did not file Form 8379, and one of them qualifies as an injured spouse, file Form 8379 by itself. Do not attach the form to Form 1040X.
However, if the couple is filing Form 1040X for an additional refund not associated with the original injured spouse claim and they do not want the injured spouse's portion of the overpayment to be applied to the offset against the spouse, then complete and attach another Form 8379 to Form 1040X. See Publication 17, Form 8379, and the Form 1040X instructions for further information.