if the instances are going to run on seperate boxes, then you don't need to do anything else other then have the seperate database instances.
if you are going to run on the same box, that is not easily supported at the moment, but you do have 1 of 2 options.
1) you could setup multiple instances of jboss to run on one machine. this would require you to setup port bindings so you don't get conflicts when trying to start multliple instances
2) you could deploy nukes multiple times in the same instance, but that would require you to change the mbean names contained in the -service.xml files so they are unique (you'd also have to change the servlet root context in the application.xml file as well).
1a) you give your machine multiple IP adresses and deploy one nukes for each
this should be done with "run -b xxx.xxx.xxx.xxx" to launch jboss on a particular network inteface. this way you can start multiple jboss one per nic