I haven't tried this specifically, but it seems like you could get it to work by using two SessionFactory objects. For instance, in your HibernateUtil class, you can have code similar to the following.
Code:
Configuration cfg = new Configuration().configure();
cfg.setProperty("hibernate.connection.user", "someuser");
cfg.setProperty("hibernate.connection.password", "somePassword");
cfg.setProperty("hibernate.connection.url", "someURL");
cfg.setProperty("hibernate.connection.driver_class", "driverForOracle7");
SessionFactory oracle7Factory = cfg.buildSessionFactory();
cfg.setProperty("hibernate.connection.user", "someuser");
cfg.setProperty("hibernate.connection.password", "somePassword");
cfg.setProperty("hibernate.connection.url", "someURL");
cfg.setProperty("hibernate.connection.driver_class", "driverForOracle9");
SessionFactory oracle9Factory = cfg.buildSessionFactory();
Then, of course, you'd have to add methods to the standard HibernateUtil so that you could get a reference to whichever SessionFactory you need at the time.