I was asked a the following question in an interview.
Can you have 2 application servers such as apache and weblogic for a single application server.
Do you mean, can you cluster 2 different application servers? Probably not, there would be no way, out of the box, to share session data between them, or to reroute requests between them. Though, if you had a load balancing server that was session aware, and would route requests for the same session to the same app server, maybe it could be done, but it's not something you'd want to do. You could however have say tomcat as a web container, with weblogic behind it providing business logic, that would work. And it's definitely possible to write applications that can be deployed to multiple application servers.