If I use the URL "/tag/tag1,tag2/feed", it will retrieve posts that have both tags. Is there anyway to do this and just get posts that have BOTH tag1 and tag2?
You could make a custom rss feed.....
Here's a link for that http://yoast.com/custom-rss-feeds-wordpress/
Simply use that info to make a page template
Make a new page through the WP interface and assign that template
The url for that page is your new feed.
You just need to adjust the query line to call to both tags
For instance, I have a hockey feed, my custom post type is col_avs, I've adjusted my query like:
$posts = query_posts('post_type=col_avs&showposts='.$numposts);
You can see it in action at http://www.rvoodoo.com/hockey-feed It goes to a feedburner feed, but that feed is generated using this method
Im not to sure if you can or can not do this. I usually just merge the two with Yahoo pipes since i use feedburner for most of my feeds check it out http://pipes.yahoo.com/pipes/