I don't have a ready made example but to get you started you will need something along the following lines:
<persistence-unit name="pctx1">
<jta-data-source>java:jboss/datasources/ExampleDS</jta-data-source>
<properties>
<property name="hibernate.connection.driver_class" value="org.hsqldb.jdbcDriver" />
<property name="hibernate.connection.url" value="jdbc:h2:mem:test1" />
<property name="hibernate.connection.username" value="sa" />
<property name="hibernate.dialect" value="org.hibernate.dialect.HSQLDialect" />
<property name="hibernate.hbm2ddl.auto" value="create" />
....
</properties>
</persistence-unit>
<persistence-unit name="pctx2">
<jta-data-source>java:jboss/datasources/ExampleDS2</jta-data-source>
<properties>
... etc
in your persistence.xml where you have configured java:jboss/datasources/ExampleDS and java:jboss/datasources/ExampleDS2 as xa datasources in your XML config
followed by
public class XAService {
@PersistenceContext(unitName = "pctx1")
EntityManager em1;
@PersistenceContext(unitName = "pctx2")
EntityManager em2;
public void doXA() {
UserTransaction ut = (UserTransaction) new InitialContext().lookup("java:comp/UserTransaction");
ut.begin();
em1.persist(new Entity1("E1"));
em2.persist(new Entity2("E2"));
ut.commit();
}