培养 = nurture, cultivate 友宜須要时间培养 Friendship needs time to nurture/cultivate 养成 = to rear and become, to form a habit, to acquire 与小人为友必养成大患 Be friends with the petty (inferior) will surely court catastrophe

Generally speaking 培養is more formal than養成.養成 is often used in two ways. One is followed by 習慣（養成良好習慣or 習慣的養成需要監督). Besides, there is 少女養成/少男養成 from Japanese originally which means someone brings up a child and trains the child to be his/her ideal lover.

培養 can be used in many more cases,e.g.培養興趣/習慣/感情（培養+something），培養孩子/子女/學生（培養+someone). It can also be used as a noun,兴趣（的）培养，人才（的）培养.

培养：to culture, to nurture. 養成：In standard Chinese if you use it as a verb, it means "get used to" . e.g. 养成习惯。It's also a noun to describe the process of getting used to something. e.g. 习惯的养成