Movable Type Tip: How to have Multiple Blogs on One RSS or Atom Feed

I have spent the last 4 plus days upgrading the from Movable Type 3.36 to Movable Type Open Source 4.2b with a complete update of templates, adding of a few new features (author archives) and a big back end information architecture re-org (only to be seen by contributors). In wanting to update the Movable Type install, I found myself trying to accomplish a few tasks that aren’t necessary in a one person blog.
One of the little things I wanted to do was to combine the RSS / Atom feeds from the main blog with the RSS / Atom Feed of the SoCal Calendar to make one feed for folks to subscribe to. When I Google searched this, I could not find any real answers, so I emailed the Six Apart Pronet list had a good simple, elegant answer from LaRosa Johnson within minutes:

“add blog_ids=”all” to the MTEntries tag of your Atom Feed and that should do it”

And I did, and it worked.
Now how did I do it? In my case, I didn’t want all the blogs on the MT install in the feed, only two. #15 and SoCal Calendar #30, so I set the blog_ids to blog_ids=”15,30″.
Everywhere in my RSS and Atom Feed templates that there was an instance of the mt:Entries tag, I added blog_ids=”15,30″, saved & published, and then tested the feeds. Happiness.
Here is an example of one the mt:Entries tag that that I altered in the Atom Feed:

<mt:Entries blog_ids=”15, 30″ lastn=”1″>

Thanks, LaRosa!