two halves of a whole; two wheels of a cart
Idiomatic expression likening two essential, interdependent elements to the two wheels of a vehicle; both are necessary for progress.
理論と実践は車の両輪だ。
Theory and practice are the two wheels of a cart.
経済成長と環境保護は車の両輪として進めるべきだ。
Economic growth and environmental protection should be advanced as two halves of a whole.
From 車 (kuruma, 'vehicle') + の (no, possessive) + 両輪 (ryōrin, 'both wheels'). The metaphor of two wheels working together to move a cart is used to describe two indispensable, complementary elements.