create separate feed that excludes posts from 2 categories?

Using the Codex section on feeds and categories, it looks like this should work to produce a feed that includes all my posts except those in either of two specified categories:

http://planetjinxatron.com/cat=-3,-15&feed=rss2

However, it does not work. Am I misinterpreting something?

I do not want to exclude the posts from my main feed. I am trying to offer a feed that’s a subset of my main feed.

Thanks for any ideas.