By default, my site’s RSS feed URLs are in this format:
category/[category-name]/feed/
As far as I can tell, that doesn’t allow me to view a feed using multiple categories. Is anyone aware of a simple workaround, or is this something I’ll need to write myself?
Maybe this might help: http://wordpress.org/support/topic/how-to-create-rss-feed-that-includes-multiple-categories
I can recommend this plugin: Feed Wrangler as it allows you to create your own custom feed but requires some modification of feed template files.
Here is an article listing 10 different external sites/services that can mix and blend feeds:
RSSMIX seems to come highly recommended it is even mentioned in the wordpress codex here: