Here is an example where feed mixing works:
Code:
class AdvancedUserRecipe1264626029(BasicNewsRecipe):
title = u'Somenews'
oldest_article = 7
max_articles_per_feed = 100
use_embedded_content = True
encoding = 'utf8'
feeds = [
(u'Kindle World', u'http://feeds2.feedburner.com/AKindleWorld')
, (u'Dilbert RSS', u'http://feeds.feedburner.com/DilbertDailyStrip')
]