A dvanda compound of words denoting (persons who have studied) subjects, which in their order of study are not remote from each other, is singular.,
As, पदकक्रमकम् 'a person who has studied the pada arrangement and a person who has studied the krama arrangement; so also क्रमकवार्त्तिकम्.
Why do we say 'order of study'? Observe पितापुत्रौ 'father and son.' Why do we say 'not remote'? See याज्ञिकवैयाकरणौ 'sacrifice-knower and the grammarian.',