나누다 generally means "to divide something already broken or a soft object by splitting it"
쪼개다 means ""to divide a hard object by breaking it"
쪼개다 cannot be used for soft object.
나무를/과일을/기업을 쪼개다.(나무,과일 and 기업 are hard object actually or conceptually)
고기를/ 빵을/ 구역을 나누다.(고기,빵 and 구역 are soft object actually or conceptually)