두 손뼉이 맞아야 소리가 난다
The palms of "two" hands have to meet to make a (clapping) sound.
Used to explain that some things cannot be done by one person alone.
손뼉 refers to the surface of the hand that is made up of both the palm of the hand (손바닥) and the fingers (손가락).
It takes two to tango.