We need both truth together all the time to stay on the Middle Way, away from the extremes of realism, idealism/nihilism, dualism/manyness and monism oneness.

Conventional truths or dependent origination taken alone as an absolute leads to the extreme of realism. Emptiness taken alone as an absolute leads to the extreme of nihilism. Thinking that everything is from a real mind only, that nothing is external, leads to the extreme of idealism / Yogacara. Etc.

We need both method and wisdom together all the time as a path because this is in accord with the fruit, with the real non-dual nature of everything. That is the meaning of the Union of The Two Truths, non-duality, inseparability of appearances and emptiness, of bliss and emptiness, of the Two Truths.