How to merge RSS feeds of two terms into one?

"The aggregator module in the base drupal install will generate RSS feeds for each taxonomy term."

Technically not correct: the Taxonomy module does the work here. You might be thinking of aggregator categories? They are separate from taxonomy terms.

"How do I make a feed that combines two terms. Two feeds into one..."

If you have two term URLs, like http://example.com/taxonomy/term/25 and http://example.com/term/35, then the combination http://example.com/taxonomy/term/25+35 will give you a page with nodes tagged with whatever term ID #25 is or #35 is (in my case, either nodes tagged with 'google' or 'china', or both). http://example.com/taxonomy/term/42,30 will give you a page with nodes tagged with both (in my case, only nodes that have both 'china' and 'google'). Add '/0/feed' to the end of the URL (so http://example.com/taxonomy/term/25,35/0/feed for both 'google' and 'china') will get you a combo feed.

This doesn't work, AFAIK, with aggregator categories, other than, it looks like, News Page (linked in the comment above).

The way I got around this limitation before was by creating a sort of hierarchy of aggregator categories:

• People
• People: Jiang Zemin
• People: Zhu Rongji

So I would categorize a feed having to do with Zhu Rongji as both "People" and "People: Zhu Rongji", so that people could subscribe to either combined aggregator feed if they wanted to.