One way to resolve your problems is to have two instances of jboss, you'll have to change all the ports of the second instance (add 1 before all ports, this way the second instance will responde in 18080).
now you can access yor sites giving the correct port or you can use apache and mod_jk and define the workers to acess the different instaces of jboss.
Thank you. But is using Apache in front of Jboss a good practice or should I use Tomcat's configuration for virtual hosts instead? What is better and why?