Example:
I am monitoring something with JSON output:

{
 "a":1,
 "b":"habracadabra",
 "c":""
}

and I want to monitor values for "a", and "b. So, I am adding JSON for preprocessing and choosing $.a for "a" element, which works fine, but when I want to add same item for monitoring "b" - there are two issues:
1. I cannot do it the same way, because Zabbix will throw the message "Item with key "blablablq" already exists"
2. Even if it will not - this behavior is inefficient, because I have to call the item twice if i have specified same period (otherwise i would have to do it several times anyway)
3. BAD workaround (well, with Zabbix, you can do everything): I can fetch entire JSON response and filter out it with calculated items, but then i have to store all the JSON data, which will be VERY inefficient due to overhead, also, calculated items will have late calculations and will have their own issues and overheads.

IMHO, good way would be:
1. To allow user to create item with the same key AND different preprocessor
2. Where updates or interval matches - do only one query instead of several