In the chandas (Vedas) the form pitarāmātarā is a valid form.,
In the ordinary language मातापितरौ is the proper form. The Vedic form is derived by adding अङ् to the first member, and आ is added to the second by 7.1.39 : and then Guna by 7.3.110. Thus आ मा गन्तां पितरामातरा च ॥,