Hand-written code :) 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.

Hand-written code :) 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= 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.