If you copy your postgres-service.xml file (in deploy directory) to another name, such as postgres-otherds-service.xml. Then change all instances of 'PostgresDS' to your new datasource name then you will have two Postgres datasources pointing at different databases.
In your jbosscmp-jdbc.xml file you have a & <datasource-mapping> in the defaults section. You need to add these (or just the datasource in your case as they are both postgres) to each item that doesn't point to the default. For example,
whatever you changed PostgresDS in the deploy dir too
As for documentation, I bought the CMP documentation from the JBoss guys. I found it more than worth the money.