Feeds are essentially WordPress Loop with customized template. So it will take regular query_posts() arguments in URL. For example see /wp-includes/feed-rss2.php template.
Since query_posts() doesn’t handle OR logic you describe it isn’t possible with URL alone.
You will have to write your own feed template that runs and concatenates two sets of post. I think it will work simply as page template, but can also be hooked into existing feed mechanics, see do_feed().
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