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.
Theory and practice are the two wheels of a cart.
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.