The affix ḍāc comes after a numeral followed by the word guṇa the sense being 'to plough so many times'.,
The word कृ is understood here. Thus द्विगुणा or त्रिगुणा करोति क्षेत्रम् \he ploughs the field twice or thrice\ = द्विगुणं विलेखनं करोति क्षेत्रस्य ॥
Why do we say \when meaning to plough\? Observe द्विगुणां करोति रज्जुम् \he doubles the rope\.,