to rely on one another for life or survival; to be mutually dependent and inseparable

to be deeply attached to one another through thick and thin; to depend on each other for survival; to stay together in life and death

literally "as close as lips and teeth"; used to describe two parties that are closely related and mutually dependent for their survival

as close as the jawbone and the cheek; used to describe a relationship where two parties are closely related and mutually dependent

interdependent; to be mutually dependent for existence

good and bad luck are closely linked and depend on each other; disaster and happiness are intertwined