According to the online docs, marking an index entry with a semicolon is supposed to produce more than one entry
The example online uses entrya;entryb - I quote the relevant section below
Code:
[blank]: If you leave the name empty, then the exact words you highlighted will be used in the index.
words: If you enter a word or phrase, then that word or phrase will be used (e.g. Oak instead of oak).
heading/words: If you use a “/” in the name then a hierarchical group will be created (e.g. Forest/Tree/Oak will create an entry Oak filed under Tree filed under Forest).
entrya;entryb: If you use a “;” then multiple entries will be created for this word or phrase.
This hasn't worked for me on 2.8.1 or 2.8.5. I just get a single index entry containing "entrya;entryb" exactly as entered. The / character does work correctly