Hah, I've recently had to solve the exact opposite problem - fetching a bunch of content from a single source, and splitting it into multiple feeds.
How did you solve this problem?
I don't split an existing RSS feed, but I scrape a website, and store the posts in a database, with a column for the user who posted it.
Then I generate an OPML file with URLs like http://localhost:1234?user=... per user, and spin up a web server that fetches only that user's feed given the query parameter. Import the OPML into the news reader, and bingo bango.