Please define the term "Parent instance". Do you mean deploying the Portal multiple times to the app server? As far as I know, that is not possible for the Portal (you can do it for other apps provided each instance has its own context, or if your define virtual hosts). Also, why would you even need to do that if you can use a single Portal instance to define multiple portals?
For setting up SSL: http://www.jboss.org/file-access/default/members/jbossas/freezone/docs/Server_Configuration_Guide/4/html/Security_on_JBoss-Using_SSL_with_JBoss_using_JSSE.html
I don't know why Google couldn't dig up these pages, I spent a long time searching. Although it shouldn't have hindered my Googling, it would be nice to carry forward some FAQ from version to version or maintain both a "General FAQ" as well as a FAQ for version-specific topics.
When I refer to "Parent Instances," it's a fairly commonly supported feature in other portal architectures (I'll name only 2 -- Dotnetnetnuke and Windows SharePoint). Within a single installation instance, websites should have full capability assigning both completely different FQDNs or as child portals of a parent portal.
The existing default JBoss installation seems to fully support child portals where you can create new portals, but each new portal by default is a virtual subdirectory/web of the base URL.
To support multiple "Parent" portals, regardless whether the new portals are subdirectories/children of the default website, each new portal can be assigned a completely different FQDN,likely a completely different registered Domain.
Another way of thinking of this is that supporting child portals only means that it's likely a single installed JBoss instance should support only one company (one FQDN) whereas if you support multiple Parent portals (different Domains) then you might deploy more than one company's website on the single JBoss instance.