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();

   }