The documentation mentions to only collect a certain set of attributes for performance reasons like for ex the Garbage Collector when doing LLD for JMX.

But if you want to create another rule to gather for ex Memory then i can make sense to add an item key like in the discovery item for the garbage collector
Ex: jmx[{#JMXJOB},{#JMXATTR}]

This will fail as zabbix will complain about the duplicate key

I think this should be allowed in LLD item prototypes as on the host items with different names will be created.