Pages with multiple inline macros try to use the same URL for the RSS feed for each inline. As a result, the last inline "wins" and overwrites the other feeds on the same page.
Josh Triplett suggests that the inline macro should take a parameter for the feed basename, and refuse to generate feeds after the first one if that parameter is not specified. That sounds like a good solution to me.
That's a reasonable fix to this longstanding bug. Autoincrementing a basename value would also work.
I've known about this bug since well, the day I wrote rss support, but I haven't seen a use case that really motivated me to take the time to fix it. Fixes or good motivation both accepted.
--Joey
A good reason to support autoincrementing might be that it's possible to have a blog feed that inlines another blog feed. On purpose, or semi-on-accident, it happened to me:
http://kitenet.net/~joey/code/whatsnew/
The result was that my whatsnew feed actually contains my Words2Nums feed, or something. --joey
I've implemented autoincrementing unique feeds, the first one on a page is a .rss, next is .rss2, etc.
There may be room for manual specification of feed basenames, but it is tricky to do that well. One problem is that if page foo adds a feed with basename bar, the resulting "foo_bar.rss" would have the same name as a feed for page foo_bar. (Assuming usedirs is not set.) This is also why I stuck the number on the end of the filename extension -- it's slightly ugly, but it avoids all such naming ambiguities.