According to the style guide we have to use, "if two or more works by different authors or authoring bodies are cited at one point in the text, use a semicolon to separate them:
(Larsen 2000; Malinowski 1999)
In some citation systems, authors’ names are presented in the text in chronological order (by date of publication); in others they are presented alphabetically. An alphabetical presentation is recommended by this manual." (Snooks & Co. 2002, Style manual for authors, editors and printers, 6th edn, John Wiley & Sons, Milton, Qld., p.193)
Is there a way to do this I’ve overlooked?
I just did a quick check and it seems that the citation style “American Psychological Association 6th edition” is doing exactly what you want. Authors will be separated by a semicolon and sorted alphabetically.
Thanks @andreas, the style I have installed in Paperpile has the requirement I’m after, it just Paperpile isn’t applying it to my document 
I have sent you a modified CSL file via our in-app messaging system that should do the trick. You need to upload it to Paperpile via: Settings -> Citation Styles -> Custom Styles. You will then see it as “Harvard - The University of Melbourne (authors sorted)”.
I will get in contact with the CSL repository guys and check with them what is going wrong here.