At the moment, SimplePie_Core is both the core feature handler as well as the feed level handler. This has a whole bunch of problems (multifeeds, e.g.) which can be solved by splitting them. See SPEP #100 for more information.

SimplePie_Core::get_items() would give the merged items as normal, however feed-level data should give a deprecated warning (but still return a value for backwards compatibility).

A new method should also be added to get the array of feeds for looping individually.