Well, one best practice is to definitely not use DefaultDS for your apps. Leave the JBoss AS services with DefaultDS (though you should use a database other than Hypersonic for it), and each of your apps should use their own data sources, unless the apps are accessing the same data in which case they would want to use the same datasource. Actually, if the two apps are accessing the same data you probably really want one set of EJBs accessing the database with two web apps using that one set of EJBs.

As to whether to run them in the same app server instance or in two instances, the answer to that depends on other factors. Such as:
* If one of the apps runs into an issue and you have to restart the app server, is that OK as far as the other app (and its users) are concerned?
* How much memory is on the host system and can it handle the large memory requirements for two app server instances?
* Would there be an issue with using two different HTTP ports to access the two applications (this can often be hidden behind and Apache HTTPD server)?

Thus you see it is more of an administration question as to which is more appropriate.