If you are using Hibernate’s proprietary API, you’ll need the hibernate.cfg.xml. If you are using JPA i.e. Hibernate EntityManager, you’ll need the persistence.xml.
So you generally don’t need both as you use either Hibernate proprietary API or JPA.
However, if you were using Hibernate Proprietary API and already have a hibernate.cfg.xml (and hbm.xml XML mapping files) but want to start using JPA, you can reuse the existing configuration files by referencing the hibernate.cfg.xml in the persistence.xml in the hibernate.ejb.cfgfile property – and thus have both files. Reusing existing hbm.xml files is IMO a realistic scenario that could justify keeping both (even if I’d probably migrate to JPA annotations on the long run).
References
- Hibernate EntityManager
- 2.2.2. Bootstrapping