How do I view a feed consisting of posts from multiple categories?

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