When I first subscribed to more than one RSS feed, my first wish was that I could just see all the articles in one place. And once I found a way of doing that, my second wish was that the articles would be arranged by date and time only and not grouped by source. Built-in browser and email rss feed readers all seemed to only have the ability to group by source.
So, I built my own website that re-orders all posts from all sources by date and time descending.