Hi HomerJ,
The short answer to that question is no, you cannot install different versions on the same server. According to the installation guide, a previously installed version would need to be removed/ uninstalled from the server prior to installing a different version. The following is occurring :
1. You will have an issue with the jboss because both application will been using the same server and it will create conflict when both users try to log in
2. Also it will overwrite the JVM files because both use different JVM settings
THe other option would be to install each version within its own VM hosted on the same server with its own individual JVM, DB, and application server.
Hope this answers your question.
Dan