(1) Joint estimated income tax payments made by married individuals who are divorced at the end of the tax year or who are legally separated at the end of a tax year for which they do not file a joint return must be split equally unless the individuals enter into a written agreement, signed by each, providing a different division. By filing a return claiming other than an equal share of joint estimated income tax payments, a taxpayer represents that a written agreement signed by both parties, as provided in this rule has been executed.

(2) Duly executed copies of the agreement are tax records and the taxpayer must retain and provide the department with copies on request.

(3) The division of jointly filed estimated payments as provided in this rule does not relieve either taxpayer from the liability for interest on any underpayment.