I think there is a way to run JBoss on different JVM's. For that, you have to change the run.bat file in jboss server bin directory replace the %JAVA_HOME% variable by absolute jdk path. Therefore it refers to user defined JDK at startup rather than environment variable.
It works for me , as i am using two jdks in my machine.
Each instance of JBoss AS that you run runs within its own JVM. As an example, we typically run multiple instances of JBoss AS simultaneously on our ES7000 servers. However, each application that is deployed to a particular instance of the JBoss AS runs within the same JVM as that instance of the server -- there is no way to run deployed applications within their own JVM. For example, if you deploy 5 applications to a single JBoss AS instance all 5 will share the JVM with each other and JBoss AS.
For scaling and tuning suggestions, you might want to look over my presentation from JBoss World: http://www.jboss.com/jbworld/int_middleware.html, look for the Scaling up JBoss presentation.
The presentation is based on information we provided for the JBoss wiki: http://www.jboss.org/wiki/Wiki.jsp?page=SPECjAppServer2002Tuning
Hope this helps.