CG-FeedRead has a well-known feature called ‘MultiFeed’ which can output to html the entries from N feeds, interleaved by date (so timestamps had best be correct!).
It also has an undisclosed feature, which I guess I’d call ‘ReFeed’, that >attempts< to formulate a simple RSS output feed from whatever it takes in. It’s a hack, not a full-featured RSS/Atom stream by any means, so wouldn’t be good for complex feeds, podcast streams, etc.. (I don’t think…). Admittedly, I coded it up initially late last year, and I haven’t touched it much since early this year, so I’ve no clue if it is currently working. ??
I don’t know offhand of other systems that will draw from multiple feeds and output a combined feed (which was why I built the feature originally), but that doesn’t mean they don’t exist!
-d