Other than a custom script, there isn’t really a faster way to do this (although you could bring them into an A4 document, set Liquid Layout to “Scale” and then change the size of the document to A5). Once the PDF has been created, Acrobat sees the spreads as single pages, so there’s not a neat, automated way to break them apart.
One suggestion would be to inform clients that there is a file preparation charge of X per page for documents not supplied as single pages. This tends to get their attention more effectively than simply saying “Please submit PDFs as single-page, not spreads.”